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Overview of Infectious Diseases Part 1
CONTINUING EDUCATION
Overview of Infectious Diseases, Part 1 by Lori J. Acuncius, PharmD, and Robert P. Henderson, PharmD Introduction
P
harmacists consistently encounter infectious disease problems in their practices. Fortunately, .the majority of infectious diseases respond to antimicrobial therapy. Pharmacists should be familiar with the antimicrobial spectrum, prophylactic and therapeutic uses, and adverse side effects associated with individual agents. Optimal use of antibiotics requires, in addition, a knowledge of each drug's pharmacokinetic characteristics and its ability to penetrate various body tissues. Recently, infectious diseases have become more complex. The list of human pathogens has been expanding not only with newly identified bacteria but also with organisms once considered to have low invasive potential. These organisms are now recognized as causes of severe infections in the immunocompromised host. The explosive development of new antibiotics, the broader capability to monitor antimicrobial drug levels, and a greater appreciation for laboratory results make the understanding of therapy complex. Antimicrobial agents are the second most commonly used class of drugs. In 1989 sales of anti-infectives in the United States totaled $4.5 billion or 16% of all prescription pharmaceutical sales. 1 In hospitals, antibiotics account for approximately 50% of total drug acquisition costs. The tremendous dollar figure, potential for adverse effects, and emergence of resistant organisms make a rational approach to the use of antimicrobial agents imperative. The first step in the rational use of antimicrobials is an understanding of infectious disease. This first of three articles discusses microbiologic characteristics and identification. It also reviews host and drug factors that must be considered when treating infection.
Considerations in Infectious Processes Treating an infection requires consideration of the organism, the host, and drug factors that might influence therapy. 2--6
Microbiological Factors The great majority of bacteria that man encounters are harmless, and some are beneficial. Bacterial pathoAmerican Pharmacy, Vol. NS30, No. 10 October 1990/611
gens are exceptions because they have developed mechanisms that allow them to invade patients and produce infectious diseases. This pathogenicity may also be referred to as virulence. Virulence may be considered in four stages: • The ability of a pathogen to gain access to the patient by surviving on or penetrating skin and mucous membranes. • The in vivo multiplication of the organism. • The inhibition or avoidance of host-protective mechanisms. • The production of disease or damage to the patient. The significance of the virulence of an organism depends on the inoculum size (infective dose) and host factors. For example, a large enough inoculum of orCE CREDIT:
To obtain one (1) hour continuing education credit for completing "Overview of Infectious Diseases, Part 1" COMPLETE:
Form 1990-CE-10 on p. 58 for grading. A certificate will be awarded upon achieving a passing grade of 70% or better. The authors of this article are Lori J. Acuncius, PharmD, and Robert P. Henderson, PharmD. Acuncius is associate professor, Samford University School of Pharmacy, Birmingham, Ala., and is affiliated with the Birmingham Veterans Administration Hospital. Henderson is associate professor, Samford University School of Pharmacy and is affiliated with the Baptist Medical Center, Birmingham. "Overview of Infectious Diseases, Part 1" is the first part of a three-part selfstudy continuing education series developed by the
American Pharmaceutical Association for pharmacists. These articles review the general principles ofinfectious diseases, including microbiology, drug therapy, and the manifestation of and recovery from infectious disease. OBJECTIVES:
Upon successful completion of this module, the pharmacist will be able to: (1~
Discuss microbiologic characteristics of microorganisms, including virulence, inoculum size, and the interplay between pathogenic organisms and normal flora.
(2) Understand host and drug factors that must be considered in treatment of infections. (3) Understand the process of microbiologic identification through specimen collection, examination, and culture and sensitivity techniques.
51
ganisms with low virulence may be able to produce disease in an otherwise healthy person. In addition, in a highly susceptible individual (i.e., immunocompromised host), small numbers of organisms usually considered to have low virulence may cause infection. The interplay between pathogenic organisms and normal flora is also important. Each body surface has a characteristic bacterial flora (normal flora) that differs from the flora of other body areas. To produce disease, the pathogenic organism must be able to survive on mucous membrane surfaces in competition with the normal flora. For some infections, the pathogen must then be able to penetrate into submucosal or deeper tissue to produce damage. Alteration of normal flora will decrease the host's ability to combat infection.
Host/Patient Factors Disorders of complement, humoral immunity, granulocytic phagocytes, and T-cell-mediated immunity will all compromise the ability of the host to fight infection. Immunoglobulins (lgG, IgM), complement, and phagocytes are important factors in the host's ability to resist and recover from bacterial infections. Cellmediated immunity is traditionally associated with protection against viral fungal disease, but the T-cell also plays a role in host defense against bacteremia. Patients with granulocytopenia or acute leukemia who develop cellulitis, pharyngitis, perirectal abscess, pneumonia, or bacteremia may be difficult to treat unless the granulocyte count and/or function returns to normal. Any decreased effectiveness of normal barriers to pathogens (intact skin and mucous membranes, mucociliary activity in the respiratory tract, enzymes and peristalsis in the gastrointestinal tract) will diminish the host's ability to fight infection. Severe burns or compound fractures may allow development of cellulitis, pneumonia, bacteremia, and osteomyelitis because of diminished integrity of the skin.
Drug Factors In choosing appropriate drug therapy, it is necessary to consider the drug's mechanism of action, pharmacokinetic properties, toxicities, and spectrum of activity. Drug factors will be discussed in Parts 2 and 3 of this series.
Initial Management of Infection Ideally, antimicrobial therapy should be initiated only if (1) significant bacterial infection is diagnosed or strongly suspected, or (2) an established indication for prophylactic antimicrobial use has been made. The bases for appropriate treatment of the patient with an infection are isolation and identification of the pathogenes) and determination of the pathogen's susceptibility to antimicrobial agents. Initial therapy with antimicrobials must often be based on assumptions, because confirming culture and sensitivity reports may be delayed for 24 to 48 hours after the patient is first evaluated. Cultures should, however, be collected before initiating therapy, if possible, to avoid inhibition of growth of the 52
Stained Smears The staining characteristics of organisms provide vital information for organism identification. The gram stain is used most frequently. Cell-wall characteristics determine whether, after decolorization, the iodine will be retained (gram-positive) or leached out (gram-negative). Counterstain often allows visualization of the numbers and types of cells or other material present. Acid-fast stains are used to identify Mycobacterium and Nocardia or Actinomyces species. Treatment for tuberculosis is usually based on acid-fast presumption of tubercle bacilli because Mycobacterium take many weeks to grow in cultures.
infecting organism. Therapy will be based on a number of parameters. Early assessment of the immune status of the patient will yield information concerning the potential etiologic agent and therapy. Individuals with a compromised immune status include renal transplant recipients, patients with acquired immunodeficiency syndrome, chronic ethanol abusers, patients with low phagocytic granulocyte counts, and those with severe nutritional deficiencies. The severity of the illness and patient stability will dictate the urgency of initial treatment. In certain patients, cultures may be obtained over a longer time frame and therapy delayed. However, prolonging the start of therapy in a patient with bacterial meningitis or gram-negative bacillary sepsis or in a febrile neutropenic patient may lead to a rapid downward course. A history of an unusual environmental or human exposure may require special laboratory/diagnostic tests and therapy to ensure adequate care. The appearance of chronic diarrhea in a patient who has recently traveled outside the United States might require special laboratory procedures to identify unusual bacteria or amoebas. The epidemiologic patterns of antibiotic resistance must be evaluated. These patterns vary among facilities and may change abruptly within an institution or geographical area, and resistance may even occur during therapy. Choosing the appropriate antibiotic agent will depend on the specific sensitivity profiles of the individual institution. For instance, a small community hospital might find that 90% of the Escherichia coli isolated by its laboratory are sensitive to gentamicin. Larger medical center hospital laboratories might have data to indicate that during the early use of gentamicin 95% of E. coli isolates were sensitive, but with heavy use, only 65% ofE. coli strains remain sensitive to the drug. Evaluation of clinical specimens (e.g., gram stain) may aid in identification of the presumed infecting agent(s) and help to direct antimicrobial therapy. Once the list of possible etiologic organisms has been narrowed, a single agent or a combination of agents American Pharmacy, Vol. NS30, No. 10 October 19901612
may be selected for empiric therapy. Before initiating antimicrobial therapy, proper specimen collection and other diagnostic testing should be completed. As more information becomes available, drug therapy may be modified.
Microbiologic Identification Specimen Collection Initial evaluation of the patient and identification of the pathogen begins, whenever possible, with collection of infected fluid or tissue. The collected material, where appropriate, should be gram stained and cultured. For reliability, specimen collection must be done properly. 2-6 Proper specimen-collection principles are listed in Table 1.
Direct Examination Gross -examination of the specimen often yields helpful information. 2,5, 7 For instance, urine and cerebrospinal fluids that are cloudy or the color of a sputum may be an initial step toward a diagnosis. Microscopic examination can provide a clue to the presence and identity of the infecting agent. In infections involving more than one pathogen, this process may help determine the relative numbers of each type of bacteria present. Staining characteristics, as well as cell morphology, may be helpful in narrowing the possible number of pathogens and in choosing therapy.
Culture and Antibiotic Susceptibility Specimens should be cultured on proper media, using appropriate technique. Identification of the organism occurs after incubation for 12 to 24 hours. From this culture, the organism may be isolated and antimicrobial susceptibility can be determined. 6 -8 A method used to determine antibiotic sensitivity is the standard disc diffusion test. In this test, organism sensitivity to an antimicrobial is correlated with the size of a zone of inhibition of growth produced by diffusion of an antibiotic from a paper disc applied to the surface of an agar plate seeded with the organism. A 16- to 18-hour incubation is generally required before test results are available (Figure 1). There are inherent limitations in this process. Only bacteriostatic activity is measured, and results are semiquantitative or are qualitative at best. Results may underpredict the usefulness of an antimicrobial agent in certain situations. The discs are impregnated with concentrations of the antimicrobial agent achievable in serum; however, an organism that is cultured from urine and·is deemed resistant by this method may in fact be susceptible to the levels of the antibiotic found in the urinary tract. An exception would occur with the discs with urinary antibacterials. Because these agents do not produce detectable serum levels, their discs are impregnated with the concentration of the drug (nalidixic acid, nitrofurantoin, norfloxacin) that is achievable in urine. Effectiveness may be overpredicted because achievable serum concentrations may not be reached in tissues where infection occurs. Also, the disc diffusion test is not useful for slow growing or fastidious organisms or for anaerobes. American Pharmacy, Vol. NS30, No. 10 October 1990/613
When it is important to obtain quantitative data or determine the bactericidal effect of an antibiotic, agar or broth dilution techniques must be used. These tests produce a quantitative value for the minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC), or both. This information may be especially helpful in serious infections that are difficult to treat, such as bacterial endocarditis and infections in the immunocompromised host. The lowest concentration of the antimicrobial agent that prevents visible growth of an organism after an 18- to 24-hour incubation period is known as the MIC. The MBC or MLC (minimum lethal concentration) may be determined in both agar and broth dilution techiques by subculturing the containers that show no growth on antibiotic-free agar-containing media. The lowest concentration of antibiotic that totally suppresses growth
Table 1. Principles of Specimen Collection 1. The specimen should be obtained from material that will most likely reflect the disease process. 2. The sample should be obtained in a manner that avoids contamination by the patient's own flora.
3. The specimen should be of sufficient volume for use, and adequate numbers collected (3 to 6) to ensure that an isolated contamination has not occurred. 4. The specimens should be collected in the proper containers and promptly delivered to the laboratory. For example, a patient with a suspected pneumonia should have several sputum collections. In many instances expectorated sputum may be adequate, while in others transtracheal aspiration is neccessary to obtain an adequate volume and to avoid contamination by normal upper respiratory (mouth) flora. If infection with an anaerobic organism is suspected, specimen collection in special containers void of oxygen would be indicated.
I
24 mm
I I
H
Bmm
Figure 1. Disc method of determining antibiotic susceptibilities. This orgl!lnism (Escherichia coli) is susceptible to K (kanamycin) and resistant to P (penicillin). From reference 9, used with permission. 53
on antibiotic-free media after overnight incubation is the MBCIMLC and shows that the organism has been killed (Figure 2). Several automated systems have been developed for rapidly determining quantitative antimicrobial susceptibility. These systems test drugs and bacteria in small volumes of broth in a fashion similar to the broth dilution technique discussed previously. Accurate results can be obtained in several hours by many systems. Published data on antimicrobial effectiveness include information on both the MIC50/90 and the MBC 50190 . MIC 50 and MBC 50 refer to the mean concentration of drug that inhibits the growth, or kills, 50% of the strains of organisms tested, respectively. Similarly, MIC 90 refers to the mean concentration of antibiotic that will inhibit the growth of 90% of the tested organisms; MBC 90 is the mean concentration required to kill 90% of the organism. Use of an antimicrobial agent that, at achievable in vivo levels, can inhibit or kill 90% of organisms would be more desirable than use of an agent that could inhibit or kill only 50% of organisms. Judicious interpretation of this type of data is necessary in assessing efficacy and toxicity. In treatment of certain infections an assay of the bactericidal effect of an antimicrobial regimen in the patient's serum may be useful. This is known as the serum bactericidal test or titer (SBT). SBT is performed by incubating a standard inoculum of the patient's organism in varying dilutions of patient serum. The serum sample is drawn at a time that correlates with either the peak or the nadir of prescribed antimicrobials (Figure 3). From studies of patients with difficult-to-treat infections or a poor clinical cure rate, an assumption can be made correlating the SBT with a successful or failing outcome. Classic clinical situations in which the SBT may be performed include bacterial endocarditis, acute and chronic osteomyelitis, and gram-negative sepsis. The SBT in Table 2 has been shown to correspond to a clinical cure or failure. lO
Other Techniques In given situations other tests may help identify the etiologic organism. These include skin testing (tuberculosis), direct immunofluorescence (syphilis), agglutination (Neisseria gonorrheae), counter-immunoelectrophoresis (Hemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis in spinal fluid). Laboratories may use enzyme techniques to rapidly identify some infectious agents (Neisseria gonorrheae, Chlamydia trachomatis, rotavirus). DNA probes have been developed that provide rapid detection or identification of specific microorganisms such as Mycoplasma pneumoniae and Legionella pneumophila. l l At present the marketed DNA probes for these microorganisms are used as screening tests. For example, although very specific, the available DNA probe for Legionella has a sensitivity of 60% to 80% and is therefore most useful as a screening test. Confirmation of infection is achieved with" 'llture or serology tests, or both. 54
Inoculum Effect The inoculum effect, a laboratory phenomenon, is said to exist when in the presence of a larger-than-standard inoculum of organisms (a standard inoculum is 1 x 104 to 5 X 105 organisms/milliliter), the MIC for a
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.
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;'.
.:-
Growth 0.25 0.5
1.0
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Interpretation MIC = 4.0 J,Lg/ml
2.0 4 .0
8.0 16.0 32.0 64 .0 J,Lg/ml
/ / /
Interpretation MBC = 8.0 J,Lg/ml
Figure 2. Determination of MIC and MBC concentrations. The minimal inhibitory concentration is defined as the lowest concentration of antibiotic that results in no visible growth (turbidity). The minimal bactericidal (lethal) concentration is the lowest concentration of antibiotic that produces a 99.9% (103) reduction in organisms from the original inoculum of approximately 105 . From reference 4, used with permission.
~~~~n~ therapy
UUulnfeCu~ingu~icro~orga~~ism~ ~ ~i~imum
Straight 1:2 seru1m
l
1:4
inhibitory concentration 1:8 1:16 1 :32 1 :64 1 :128 Culture
1 1 1.
r v v r v y };' ) ~)
480
Broth
Minimum lethal concentration
Figure 3. The test for serum antimicrobial activity determines the maximum dilution of serum that will show bacteriostatic or bactericidal (lethal) activity against an infecting organism. Interpretation of results: Bacteriostatic at 1 :16 dilution, bactericidal at 1:8 dilution. (Adapted from Youmans Gp, Patterson PY, and Sommers H. The Biologic and Clinical Basis of Infectious Diseases. Philadelphia: WB Saunders; 1975.) From reference 4, used with permission.
Table 2. Classic Clinical Situations for Use of Serum Bactericidal Tests Disease State
Cure
Bacterial endocarditis
Peak
Osteomyelitis Acute Chronic Gram-negative bacteremia Granulocytes> 1000
Peak~1:16
Granulocytes <100
Failure ~1
:32
Peak
~1 : 8
Peak
~1
Nadir<1 :2 Nadir <1:4
:16
American Pharmacy, Vol. NS30, No. 10 October 1990/614
particular drug is significantly increased. 12 The inoculum effect occurs with specific antimicrobials and specific organisms. For example, antibiotics sensitive to beta-lactamase hydrolysis (cefazolin, ampicillin, penicillin, cefaclor) will exhibit the inoculum effect with penicillinase-producing Staphylococcus aureus. In contrast, antimicrobials resistant to beta-lactamase (quinolones, aminoglycosides, vancomycin) do not display this phenomenon. The Enterobacteriaceae and Pseudomonas aeruginosa have been shown to produce the inoculum effect when treated with most third-generation cephalosporins. The inoculum effect does not appear to be significant with gram-negative bacilli and the quinolones, aminoglycosides, or imipenem. The clinical implications of this laboratory phenomenon are unclear. This effect might be of importance in situations where large numbers of organisms are contained in a relatively dormant state (abscess, diabetic foot ulcer). However, even with the inoculum effect, a favorable response to therapy may result if the MIC can be maintained at 8 times lower than clinically achievable serum/tissue levels of the drug used. Future studies will be necessary to draw any correlation between the inoculum effect and clinical response.
Summary Microbiologic characteristics include virulence, inoculum size, and the interplay between normal flora and pathogenic organisms. These characteristics, as well as relevant host factors, are an important part of the infectious disease process. Parts 2 and 3 of this
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1. Professional Communication. Pharmaceutical Data Services. Phoenix, Az; June 27, 1990. 2. Moellering RC. Principles of anti-infective therapy. In: Mandell GL, Douglas Jr RG, Bennett JE, eds. Principles and Practice of Infectious Diseases. 3rd ed. New York: Churchill Livingstone; 1990:206-16. 3. Sanders WE. Antimicrobial therapy. In: Cleef LE, Johnson IV JE, eds. Clinical Concepts of Infectious Diseases. 3rd ed. Baltimore, Md: Williams & Wilkins; 1982:277-89. 4. Conte JE, Barriere SL. Manual ofAntibiotics and Infectious Diseases. 5th ed. Philadelphia: Lea & Febiger; 1981. 5. Eliopoulas GM, Moellering RC. Principles of antibiotic therapy. Med Clin North Am. 1982;66(1):3-15. 6. Wilkowske CJ, Hermans PE. General principles of antimicrobial therapy. Mayo Clin Proc. 1987;62:689-98. 7. Ellner PD. Diagnostic laboratory procedures in infectious diseases. Med Clin North Am. 1987;71(6):1065-79. 8. Rosenblatt JE. Laboratory tests used to guide antimicrobial therapy. Mayo Clin Proc. 1987;62:799-805. 9. Fischbach F. Sensitivity of bacteria to antimicrobial agents. A Manual of Laboratory Diagnostic Tests. 2nd ed. Philadelphia: JB Lippincott Co; 1984:367. 10. Vosti K. Serum bactericidal test: past, present, and future use in the management of patients with infections. Curr Clin Topics Infect Dis. 1989;10:43-55. 11. Washington IN. Bacteria, fungi, and parasites. In: Mandell GL, Douglas Jr RG, Bennett JE, eds. Principles and Practice ofInfectious Diseases. 3rd ed. New York: Churchill Livingstone; 1990:179. 12. Brook!. Inoculum effect. Rev InfectDis. 1989;2(3):361-68.
Instructions: For each question, blacken the letter corresponding to the answer you select as being the correct one. Please review all your answers to be sure you have blackened the proper spaces. There is only one correct answer.
When Medicine Is Prescribed or Dispensed Be Sure To Ask:
~ The National Council on Patient Information and Education
References
Form 1990-CE-I0 ASSESSMENT EXERCISE
Americans want more information about their medicines, but aren't sure what to ask ...
•
series will review the use of drug therapy and the general manifestations of infectious diseases.
@
TAL~OUT
PRESCRIPTIONS
American Pharmacy, Vol. NS30, No. 10 October 1990/615
1. Which of the following is TRUE concerning microbiological factors of an infection? a. In order for a microorganism to produce infection, it must penetrate or survive on skin and mucous membranes, multiply in vivo, inhibit or avoid host-protective mechanisms, and produce disease in the patient. b. A large enough inoculum (infective dose) of an organism with low virulence may be able to produce disease in an otherwise healthy individual. c. Normal flora always increase the ability of bacterial pathogens to produce disease. d. A and B. e. All of the above. 2. Which of the following is TRUE concerning host factors in infectious diseases? a. Patients with granulocytopenia have an increased risk of developing bacterial infections. b. Severe burns or trauma alter the skin's protective barrier and may predispose the patient to infections. c. Decreased cell-mediated immunity may predispose the patient to viral, tungal, and bacterial infections. d. A and B. e. All of the above. CE (continued on p. 58)
55
Overview of Infectious Diseases, Part 1 by Lori J. Acuncius, PharmD, and Robert P. Henderson, PharmD Introduction
P
harmacists consistently encounter infectious disease problems in their practices. Fortunately, .the majority of infectious diseases respond to antimicrobial therapy. Pharmacists should be familiar with the antimicrobial spectrum, prophylactic and therapeutic uses, and adverse side effects associated with individual agents. Optimal use of antibiotics requires, in addition, a knowledge of each drug's pharmacokinetic characteristics and its ability to penetrate various body tissues. Recently, infectious diseases have become more complex. The list of human pathogens has been expanding not only with newly identified bacteria but also with organisms once considered to have low invasive potential. These organisms are now recognized as causes of severe infections in the immunocompromised host. The explosive development of new antibiotics, the broader capability to monitor antimicrobial drug levels, and a greater appreciation for laboratory results make the understanding of therapy complex. Antimicrobial agents are the second most commonly used class of drugs. In 1989 sales of anti-infectives in the United States totaled $4.5 billion or 16% of all prescription pharmaceutical sales. 1 In hospitals, antibiotics account for approximately 50% of total drug acquisition costs. The tremendous dollar figure, potential for adverse effects, and emergence of resistant organisms make a rational approach to the use of antimicrobial agents imperative. The first step in the rational use of antimicrobials is an understanding of infectious disease. This first of three articles discusses microbiologic characteristics and identification. It also reviews host and drug factors that must be considered when treating infection.
Considerations in Infectious Processes Treating an infection requires consideration of the organism, the host, and drug factors that might influence therapy. 2--6
Microbiological Factors The great majority of bacteria that man encounters are harmless, and some are beneficial. Bacterial pathoAmerican Pharmacy, Vol. NS30, No. 10 October 1990/611
gens are exceptions because they have developed mechanisms that allow them to invade patients and produce infectious diseases. This pathogenicity may also be referred to as virulence. Virulence may be considered in four stages: • The ability of a pathogen to gain access to the patient by surviving on or penetrating skin and mucous membranes. • The in vivo multiplication of the organism. • The inhibition or avoidance of host-protective mechanisms. • The production of disease or damage to the patient. The significance of the virulence of an organism depends on the inoculum size (infective dose) and host factors. For example, a large enough inoculum of orCE CREDIT:
To obtain one (1) hour continuing education credit for completing "Overview of Infectious Diseases, Part 1" COMPLETE:
Form 1990-CE-10 on p. 58 for grading. A certificate will be awarded upon achieving a passing grade of 70% or better. The authors of this article are Lori J. Acuncius, PharmD, and Robert P. Henderson, PharmD. Acuncius is associate professor, Samford University School of Pharmacy, Birmingham, Ala., and is affiliated with the Birmingham Veterans Administration Hospital. Henderson is associate professor, Samford University School of Pharmacy and is affiliated with the Baptist Medical Center, Birmingham. "Overview of Infectious Diseases, Part 1" is the first part of a three-part selfstudy continuing education series developed by the
American Pharmaceutical Association for pharmacists. These articles review the general principles ofinfectious diseases, including microbiology, drug therapy, and the manifestation of and recovery from infectious disease. OBJECTIVES:
Upon successful completion of this module, the pharmacist will be able to: (1~
Discuss microbiologic characteristics of microorganisms, including virulence, inoculum size, and the interplay between pathogenic organisms and normal flora.
(2) Understand host and drug factors that must be considered in treatment of infections. (3) Understand the process of microbiologic identification through specimen collection, examination, and culture and sensitivity techniques.
51
ganisms with low virulence may be able to produce disease in an otherwise healthy person. In addition, in a highly susceptible individual (i.e., immunocompromised host), small numbers of organisms usually considered to have low virulence may cause infection. The interplay between pathogenic organisms and normal flora is also important. Each body surface has a characteristic bacterial flora (normal flora) that differs from the flora of other body areas. To produce disease, the pathogenic organism must be able to survive on mucous membrane surfaces in competition with the normal flora. For some infections, the pathogen must then be able to penetrate into submucosal or deeper tissue to produce damage. Alteration of normal flora will decrease the host's ability to combat infection.
Host/Patient Factors Disorders of complement, humoral immunity, granulocytic phagocytes, and T-cell-mediated immunity will all compromise the ability of the host to fight infection. Immunoglobulins (lgG, IgM), complement, and phagocytes are important factors in the host's ability to resist and recover from bacterial infections. Cellmediated immunity is traditionally associated with protection against viral fungal disease, but the T-cell also plays a role in host defense against bacteremia. Patients with granulocytopenia or acute leukemia who develop cellulitis, pharyngitis, perirectal abscess, pneumonia, or bacteremia may be difficult to treat unless the granulocyte count and/or function returns to normal. Any decreased effectiveness of normal barriers to pathogens (intact skin and mucous membranes, mucociliary activity in the respiratory tract, enzymes and peristalsis in the gastrointestinal tract) will diminish the host's ability to fight infection. Severe burns or compound fractures may allow development of cellulitis, pneumonia, bacteremia, and osteomyelitis because of diminished integrity of the skin.
Drug Factors In choosing appropriate drug therapy, it is necessary to consider the drug's mechanism of action, pharmacokinetic properties, toxicities, and spectrum of activity. Drug factors will be discussed in Parts 2 and 3 of this series.
Initial Management of Infection Ideally, antimicrobial therapy should be initiated only if (1) significant bacterial infection is diagnosed or strongly suspected, or (2) an established indication for prophylactic antimicrobial use has been made. The bases for appropriate treatment of the patient with an infection are isolation and identification of the pathogenes) and determination of the pathogen's susceptibility to antimicrobial agents. Initial therapy with antimicrobials must often be based on assumptions, because confirming culture and sensitivity reports may be delayed for 24 to 48 hours after the patient is first evaluated. Cultures should, however, be collected before initiating therapy, if possible, to avoid inhibition of growth of the 52
Stained Smears The staining characteristics of organisms provide vital information for organism identification. The gram stain is used most frequently. Cell-wall characteristics determine whether, after decolorization, the iodine will be retained (gram-positive) or leached out (gram-negative). Counterstain often allows visualization of the numbers and types of cells or other material present. Acid-fast stains are used to identify Mycobacterium and Nocardia or Actinomyces species. Treatment for tuberculosis is usually based on acid-fast presumption of tubercle bacilli because Mycobacterium take many weeks to grow in cultures.
infecting organism. Therapy will be based on a number of parameters. Early assessment of the immune status of the patient will yield information concerning the potential etiologic agent and therapy. Individuals with a compromised immune status include renal transplant recipients, patients with acquired immunodeficiency syndrome, chronic ethanol abusers, patients with low phagocytic granulocyte counts, and those with severe nutritional deficiencies. The severity of the illness and patient stability will dictate the urgency of initial treatment. In certain patients, cultures may be obtained over a longer time frame and therapy delayed. However, prolonging the start of therapy in a patient with bacterial meningitis or gram-negative bacillary sepsis or in a febrile neutropenic patient may lead to a rapid downward course. A history of an unusual environmental or human exposure may require special laboratory/diagnostic tests and therapy to ensure adequate care. The appearance of chronic diarrhea in a patient who has recently traveled outside the United States might require special laboratory procedures to identify unusual bacteria or amoebas. The epidemiologic patterns of antibiotic resistance must be evaluated. These patterns vary among facilities and may change abruptly within an institution or geographical area, and resistance may even occur during therapy. Choosing the appropriate antibiotic agent will depend on the specific sensitivity profiles of the individual institution. For instance, a small community hospital might find that 90% of the Escherichia coli isolated by its laboratory are sensitive to gentamicin. Larger medical center hospital laboratories might have data to indicate that during the early use of gentamicin 95% of E. coli isolates were sensitive, but with heavy use, only 65% ofE. coli strains remain sensitive to the drug. Evaluation of clinical specimens (e.g., gram stain) may aid in identification of the presumed infecting agent(s) and help to direct antimicrobial therapy. Once the list of possible etiologic organisms has been narrowed, a single agent or a combination of agents American Pharmacy, Vol. NS30, No. 10 October 19901612
may be selected for empiric therapy. Before initiating antimicrobial therapy, proper specimen collection and other diagnostic testing should be completed. As more information becomes available, drug therapy may be modified.
Microbiologic Identification Specimen Collection Initial evaluation of the patient and identification of the pathogen begins, whenever possible, with collection of infected fluid or tissue. The collected material, where appropriate, should be gram stained and cultured. For reliability, specimen collection must be done properly. 2-6 Proper specimen-collection principles are listed in Table 1.
Direct Examination Gross -examination of the specimen often yields helpful information. 2,5, 7 For instance, urine and cerebrospinal fluids that are cloudy or the color of a sputum may be an initial step toward a diagnosis. Microscopic examination can provide a clue to the presence and identity of the infecting agent. In infections involving more than one pathogen, this process may help determine the relative numbers of each type of bacteria present. Staining characteristics, as well as cell morphology, may be helpful in narrowing the possible number of pathogens and in choosing therapy.
Culture and Antibiotic Susceptibility Specimens should be cultured on proper media, using appropriate technique. Identification of the organism occurs after incubation for 12 to 24 hours. From this culture, the organism may be isolated and antimicrobial susceptibility can be determined. 6 -8 A method used to determine antibiotic sensitivity is the standard disc diffusion test. In this test, organism sensitivity to an antimicrobial is correlated with the size of a zone of inhibition of growth produced by diffusion of an antibiotic from a paper disc applied to the surface of an agar plate seeded with the organism. A 16- to 18-hour incubation is generally required before test results are available (Figure 1). There are inherent limitations in this process. Only bacteriostatic activity is measured, and results are semiquantitative or are qualitative at best. Results may underpredict the usefulness of an antimicrobial agent in certain situations. The discs are impregnated with concentrations of the antimicrobial agent achievable in serum; however, an organism that is cultured from urine and·is deemed resistant by this method may in fact be susceptible to the levels of the antibiotic found in the urinary tract. An exception would occur with the discs with urinary antibacterials. Because these agents do not produce detectable serum levels, their discs are impregnated with the concentration of the drug (nalidixic acid, nitrofurantoin, norfloxacin) that is achievable in urine. Effectiveness may be overpredicted because achievable serum concentrations may not be reached in tissues where infection occurs. Also, the disc diffusion test is not useful for slow growing or fastidious organisms or for anaerobes. American Pharmacy, Vol. NS30, No. 10 October 1990/613
When it is important to obtain quantitative data or determine the bactericidal effect of an antibiotic, agar or broth dilution techniques must be used. These tests produce a quantitative value for the minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC), or both. This information may be especially helpful in serious infections that are difficult to treat, such as bacterial endocarditis and infections in the immunocompromised host. The lowest concentration of the antimicrobial agent that prevents visible growth of an organism after an 18- to 24-hour incubation period is known as the MIC. The MBC or MLC (minimum lethal concentration) may be determined in both agar and broth dilution techiques by subculturing the containers that show no growth on antibiotic-free agar-containing media. The lowest concentration of antibiotic that totally suppresses growth
Table 1. Principles of Specimen Collection 1. The specimen should be obtained from material that will most likely reflect the disease process. 2. The sample should be obtained in a manner that avoids contamination by the patient's own flora.
3. The specimen should be of sufficient volume for use, and adequate numbers collected (3 to 6) to ensure that an isolated contamination has not occurred. 4. The specimens should be collected in the proper containers and promptly delivered to the laboratory. For example, a patient with a suspected pneumonia should have several sputum collections. In many instances expectorated sputum may be adequate, while in others transtracheal aspiration is neccessary to obtain an adequate volume and to avoid contamination by normal upper respiratory (mouth) flora. If infection with an anaerobic organism is suspected, specimen collection in special containers void of oxygen would be indicated.
I
24 mm
I I
H
Bmm
Figure 1. Disc method of determining antibiotic susceptibilities. This orgl!lnism (Escherichia coli) is susceptible to K (kanamycin) and resistant to P (penicillin). From reference 9, used with permission. 53
on antibiotic-free media after overnight incubation is the MBCIMLC and shows that the organism has been killed (Figure 2). Several automated systems have been developed for rapidly determining quantitative antimicrobial susceptibility. These systems test drugs and bacteria in small volumes of broth in a fashion similar to the broth dilution technique discussed previously. Accurate results can be obtained in several hours by many systems. Published data on antimicrobial effectiveness include information on both the MIC50/90 and the MBC 50190 . MIC 50 and MBC 50 refer to the mean concentration of drug that inhibits the growth, or kills, 50% of the strains of organisms tested, respectively. Similarly, MIC 90 refers to the mean concentration of antibiotic that will inhibit the growth of 90% of the tested organisms; MBC 90 is the mean concentration required to kill 90% of the organism. Use of an antimicrobial agent that, at achievable in vivo levels, can inhibit or kill 90% of organisms would be more desirable than use of an agent that could inhibit or kill only 50% of organisms. Judicious interpretation of this type of data is necessary in assessing efficacy and toxicity. In treatment of certain infections an assay of the bactericidal effect of an antimicrobial regimen in the patient's serum may be useful. This is known as the serum bactericidal test or titer (SBT). SBT is performed by incubating a standard inoculum of the patient's organism in varying dilutions of patient serum. The serum sample is drawn at a time that correlates with either the peak or the nadir of prescribed antimicrobials (Figure 3). From studies of patients with difficult-to-treat infections or a poor clinical cure rate, an assumption can be made correlating the SBT with a successful or failing outcome. Classic clinical situations in which the SBT may be performed include bacterial endocarditis, acute and chronic osteomyelitis, and gram-negative sepsis. The SBT in Table 2 has been shown to correspond to a clinical cure or failure. lO
Other Techniques In given situations other tests may help identify the etiologic organism. These include skin testing (tuberculosis), direct immunofluorescence (syphilis), agglutination (Neisseria gonorrheae), counter-immunoelectrophoresis (Hemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis in spinal fluid). Laboratories may use enzyme techniques to rapidly identify some infectious agents (Neisseria gonorrheae, Chlamydia trachomatis, rotavirus). DNA probes have been developed that provide rapid detection or identification of specific microorganisms such as Mycoplasma pneumoniae and Legionella pneumophila. l l At present the marketed DNA probes for these microorganisms are used as screening tests. For example, although very specific, the available DNA probe for Legionella has a sensitivity of 60% to 80% and is therefore most useful as a screening test. Confirmation of infection is achieved with" 'llture or serology tests, or both. 54
Inoculum Effect The inoculum effect, a laboratory phenomenon, is said to exist when in the presence of a larger-than-standard inoculum of organisms (a standard inoculum is 1 x 104 to 5 X 105 organisms/milliliter), the MIC for a
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8.0 16.0 32.0 64 .0 J,Lg/ml
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Interpretation MBC = 8.0 J,Lg/ml
Figure 2. Determination of MIC and MBC concentrations. The minimal inhibitory concentration is defined as the lowest concentration of antibiotic that results in no visible growth (turbidity). The minimal bactericidal (lethal) concentration is the lowest concentration of antibiotic that produces a 99.9% (103) reduction in organisms from the original inoculum of approximately 105 . From reference 4, used with permission.
~~~~n~ therapy
UUulnfeCu~ingu~icro~orga~~ism~ ~ ~i~imum
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l
1:4
inhibitory concentration 1:8 1:16 1 :32 1 :64 1 :128 Culture
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Figure 3. The test for serum antimicrobial activity determines the maximum dilution of serum that will show bacteriostatic or bactericidal (lethal) activity against an infecting organism. Interpretation of results: Bacteriostatic at 1 :16 dilution, bactericidal at 1:8 dilution. (Adapted from Youmans Gp, Patterson PY, and Sommers H. The Biologic and Clinical Basis of Infectious Diseases. Philadelphia: WB Saunders; 1975.) From reference 4, used with permission.
Table 2. Classic Clinical Situations for Use of Serum Bactericidal Tests Disease State
Cure
Bacterial endocarditis
Peak
Osteomyelitis Acute Chronic Gram-negative bacteremia Granulocytes> 1000
Peak~1:16
Granulocytes <100
Failure ~1
:32
Peak
~1 : 8
Peak
~1
Nadir<1 :2 Nadir <1:4
:16
American Pharmacy, Vol. NS30, No. 10 October 1990/614
particular drug is significantly increased. 12 The inoculum effect occurs with specific antimicrobials and specific organisms. For example, antibiotics sensitive to beta-lactamase hydrolysis (cefazolin, ampicillin, penicillin, cefaclor) will exhibit the inoculum effect with penicillinase-producing Staphylococcus aureus. In contrast, antimicrobials resistant to beta-lactamase (quinolones, aminoglycosides, vancomycin) do not display this phenomenon. The Enterobacteriaceae and Pseudomonas aeruginosa have been shown to produce the inoculum effect when treated with most third-generation cephalosporins. The inoculum effect does not appear to be significant with gram-negative bacilli and the quinolones, aminoglycosides, or imipenem. The clinical implications of this laboratory phenomenon are unclear. This effect might be of importance in situations where large numbers of organisms are contained in a relatively dormant state (abscess, diabetic foot ulcer). However, even with the inoculum effect, a favorable response to therapy may result if the MIC can be maintained at 8 times lower than clinically achievable serum/tissue levels of the drug used. Future studies will be necessary to draw any correlation between the inoculum effect and clinical response.
Summary Microbiologic characteristics include virulence, inoculum size, and the interplay between normal flora and pathogenic organisms. These characteristics, as well as relevant host factors, are an important part of the infectious disease process. Parts 2 and 3 of this
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WHAT THE MEDICINE IS SUPPOSED TO DO; v' HOW AND WHEN TO TAKE IT, AND FOR HOW LONG; v' WHAT TO DO IF SIDE EFFECTS OCCUR; v' WHAT FOODS, DRINKS, OTHER MEDICINES OR ACTIVITIES SHOULD BE AVOIDED WHILE TAKING THE MEDICINE; v' IS THERE WRITTEN INFORMATION AVAILABLE ABOUT THE MEDICINE? v'
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1. Professional Communication. Pharmaceutical Data Services. Phoenix, Az; June 27, 1990. 2. Moellering RC. Principles of anti-infective therapy. In: Mandell GL, Douglas Jr RG, Bennett JE, eds. Principles and Practice of Infectious Diseases. 3rd ed. New York: Churchill Livingstone; 1990:206-16. 3. Sanders WE. Antimicrobial therapy. In: Cleef LE, Johnson IV JE, eds. Clinical Concepts of Infectious Diseases. 3rd ed. Baltimore, Md: Williams & Wilkins; 1982:277-89. 4. Conte JE, Barriere SL. Manual ofAntibiotics and Infectious Diseases. 5th ed. Philadelphia: Lea & Febiger; 1981. 5. Eliopoulas GM, Moellering RC. Principles of antibiotic therapy. Med Clin North Am. 1982;66(1):3-15. 6. Wilkowske CJ, Hermans PE. General principles of antimicrobial therapy. Mayo Clin Proc. 1987;62:689-98. 7. Ellner PD. Diagnostic laboratory procedures in infectious diseases. Med Clin North Am. 1987;71(6):1065-79. 8. Rosenblatt JE. Laboratory tests used to guide antimicrobial therapy. Mayo Clin Proc. 1987;62:799-805. 9. Fischbach F. Sensitivity of bacteria to antimicrobial agents. A Manual of Laboratory Diagnostic Tests. 2nd ed. Philadelphia: JB Lippincott Co; 1984:367. 10. Vosti K. Serum bactericidal test: past, present, and future use in the management of patients with infections. Curr Clin Topics Infect Dis. 1989;10:43-55. 11. Washington IN. Bacteria, fungi, and parasites. In: Mandell GL, Douglas Jr RG, Bennett JE, eds. Principles and Practice ofInfectious Diseases. 3rd ed. New York: Churchill Livingstone; 1990:179. 12. Brook!. Inoculum effect. Rev InfectDis. 1989;2(3):361-68.
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References
Form 1990-CE-I0 ASSESSMENT EXERCISE
Americans want more information about their medicines, but aren't sure what to ask ...
•
series will review the use of drug therapy and the general manifestations of infectious diseases.
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TAL~OUT
PRESCRIPTIONS
American Pharmacy, Vol. NS30, No. 10 October 1990/615
1. Which of the following is TRUE concerning microbiological factors of an infection? a. In order for a microorganism to produce infection, it must penetrate or survive on skin and mucous membranes, multiply in vivo, inhibit or avoid host-protective mechanisms, and produce disease in the patient. b. A large enough inoculum (infective dose) of an organism with low virulence may be able to produce disease in an otherwise healthy individual. c. Normal flora always increase the ability of bacterial pathogens to produce disease. d. A and B. e. All of the above. 2. Which of the following is TRUE concerning host factors in infectious diseases? a. Patients with granulocytopenia have an increased risk of developing bacterial infections. b. Severe burns or trauma alter the skin's protective barrier and may predispose the patient to infections. c. Decreased cell-mediated immunity may predispose the patient to viral, tungal, and bacterial infections. d. A and B. e. All of the above. CE (continued on p. 58)
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