- Email: [email protected]
Comparison of mupirocin susceptibility of nasal and nonnasal Staphylococcus aureus isolates
DIAGN MICROBIOLINFECTDIS 1994;20:171-174
171
Comparison of Mupirocin Susceptibility of Nasal and Nonnasal Staphylococcus aureus Isolates Linda J. Utrup, Jane E. Finlay, Stephen F. Rittenhouse, and James A. Poupard
Susceptibilities of 414 nasal and 586 nonnasal Staphylococcus aureus isolates, both methiciUin resistant and methicillin susceptible, to the topical antimicrobial agent mupirocin were compared. A susceptibility of 99.1% was observed for the I000
isolates. Nasal and nonnasal isolates showed a similar 90% minimum inhibitory concentration (MIC9o) and statistically equivalent percent susceptibilities.
The incidence of nosocomial staphylococcal infections and outbreaks is increasing worldwide (Boyce, 1989; Haley et al., 1982), and in many countries, including the United States, staphylococci have emerged as major hospital pathogens. The emergence of methicillin-resistant Staphylococcus aureus (MRSA) has created a significant dilemma, because therapeutic choices are limited. Rates of MRSA infections and colonization are increasing in medical center, community hospital, and nursing home patients (Boyce, 1989; Haley et al., 1982; Mylotte et al., 1992; Panlilio et al., 1992; Ward and Stausbaugh, 1992). The control of staphylococcal nosocomial transmission is important because some MRSA strains have the ability to spread rapidly and cause considerable morbidity and mortality (Boyce, 1992). Once MRSA is introduced into a hospital or nursing home, eradication can be difficult (Boyce, 1981).
In long-term care facilities, patients and healthcare workers may carry staphylococci in their nares or on their hands, and some patients become infected after they are colonized (Boyce, 1989; Thompson et al., 1982). Reservoirs such as the nares allow staphylococci to exchange genetic material with drug-resistant organisms or to persist and be exposed to the selective pressures that lead to the development of resistance. Eradication of the nasal carriage state in both patients and health-care workers is important in controlling staphylococcal infections. Topical antimicrobials are particularly beneficial in eliminating staphylococcal nasal carriage, while reducing the possibility of toxicity associated with systemic agents (Wenzel et al., 1991). Mupirocin, the approved generic name for pseudomonic acid A, a fermentation product of Pseudomonas fluorescens NCIB 10586, has the unique mode of action of inhibiting bacterial isoleucyl tRNA synthetase, thus blocking translation and protein synthesis (Hughes and Mellows, 1978). Mupirocin lacks cross resistance with other antibacterial agents and exhibits activity against strains of bacteria that are multiply drug resistant (Pappa, 1990; Sutherland et al., 1985). Mupirocin in a polyethylene glycol base is approved for use in most countries, including the United States, for treatment of impetigo caused by staphylococci and streptococci. A mupirocin nasal formulation in a white, soft paraffin and lanolin ointment is approved in Europe, and is undergoing clinical
From the Department of Microbial Cell Sciences, Clinical Microbiology, SmithKline Beecham Pharmaceuticals, King of Prussia, Pennsylvania, USA. L.J. Utrup is currently employed at a different institution. Data on 230 isolates were previously published in ICAAC Poster 1141, 1992. Address reprint requests to J. Finlay, SmithKline Beecham Pharmaceuticals, PO Box 1539, King'of Prussia, PA 19406, USA. Received 26 July 1994; revised and accepted 6 September 1994. © 1994Elsevier Science Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893/94/$7.00
172
trials in the United States. Clinical studies have shown that the mupirocin nasal formulation is safe and effective for use against MRSA and ~-lactamase-producing staphylococcal isolates carried in the nares (Casewell and Hill, 1986; Redhead et al., 1991; Wenzel et al., 1991). Because mupirocin has been used in the treatment of impetigo in the United States for a number of years, it was determined that a comparison of susceptibility results for nasal and nonnasal isolates would be useful. The purpose of this study was to compare the mupirocin susceptibilities of recent nasal and nonnasal isolates of S. aureus. A total of 1000 S. aureus isolates were included in the study: 414 were nasal and 586 were nonnasal. All isolates were identified as S. aureus, and most were derived from individual patients. Of the nasal isolates, 246 were obtained from a multicenter, US clinical study evaluating the safety, tolerance, and efficacy of calcium mupirocin, and 57 isolates were obtained from a Canadian clinical study evaluating the efficacy of calcium mupirocin in eradicating the carrier state of S. aureus. Only pretreatment isolates from the clinical studies were included in this evaluation. Dr. Stephen Brecher of the Boston Veterans Administration Hospital, Boston, Massachusetts, provided 94 nasal isolates from his collection, most of which were MRSA. Another nine nasal isolates were collected from L. Steele Moore of the Medical Center of Delaware, Wilmington, Delaware, and eight nasal isolates were obtained from SmithKline Beecham Clinical Laboratories (Philadelphia, PA, USA). Of the 586 nonnasal S. aureus isolates, 112 were provided by Dr. Julio Ramirez of the University of Louisville, Louisville, Kentucky, and the remainder were obtained from SmithKline Beecham Clinical Laboratories. Previous exposure of the isolates to mupirocin is unknown; however, all isolates were collected since the introduction of mupirocin in the United States. Organisms were isolated on 5% sheep blood agar plates (Becton Dickinson Microbiology Systems, Cockeysville, MD, USA) and stored at -70°C in trypticase soy broth with 20% glycerol (Becton Dickinson Microbiology Systems). Staphylococcus aureus isolates were identified by the Staph Ident System (bioMerieux Vitek) along with tube coagulase (Becton Dickinson Microbiology Systems) and catalase (Mallinckrodt AR, Paris, KY, USA) tests. Methicillin resistance was determined by using MRSA Screen Agar (Becton Dickinson Microbiology Systems) according to the recommended procedure of the manufacturer. Minimum inhibitory concentration (MIC) determinations were performed according to National Committee for Clinical Laboratory Standards recommended methods (NCCLS, 1993). Microtiter MIC
L.J. Utrup et al.
trays were prepared in-house employing an automatic dispensing system (Quick Spense lie; Sandy Spring Instrument Company, Germantown, MD, USA). Trays were stored at -70°C. All wells contained 0.05 ml of antibiotic solution in doubling dilutions. Mupirocin (SmithKline Beecham) was dissolved in water. The MIC trays were thawed and inoculated with an equal volume, 0.05 ml, of a 1:100 dilution of a 0.5 McFarland adjusted culture so that the final organism concentration in the microtiter wells was 5 x 10s colony-forming units/ml. The McFarland adjusted culture was prepared from four to five colonies grown at 35°-37°C for 18-24 h on 5% sheep blood agar plates (Becton Dickinson Microbiology Systems). Microtiter trays were incubated for 18-24 h at 35°-37°C. The MICs were read as the lowest concentration of mupirocin required to inhibit visible growth of the organisms. A break point of 4 4 ~g/ml was used. This break point was determined following an evaluation of the interpretive criteria for mupirocin susceptibility testing using a large number of staphylococci, including strains with high MICs. Major errors were minimized by using an MIC break point of 4 4 ~g/ml and a 5-~g disk break point of ~18 mm. These break points were used in the clinical trials from which most of the isolates were obtained. The distribution of the 1000 isolates, including the number of nasal and nonnasal isolates, and the methicillin susceptibility of the isolates are listed in Table 1: 41% of the isolates were nasal, 59% were nonnasal, and 22% were MRSA. Table 1 also demonstrates the characteristics of the nasal and nonnasal isolates. There was an approximately even distribution of MRSA b e t w e e n the two groups of isolates. A comparison of the 1000 S. aureus isolates by MICs0, MIC90, MIC ranges, the number of organisms with MICs >4 ~g/ml, and the percent susceptible organisms are presented in Table 2. The MICs0 for each group of isolates was 0.25 ~g/ml. The MICg0 for the 1000 isolates and for the nasal isolates was 0.5 ~g/ml, whereas the nonnasal isolate MICg0 was 0.25 ~g/ml. Isolates that were found to be MRSA had an MICg0 of 0.5 ~g/ml, whereas methicillin susceptible S. aureus (MSSA) isolates demonstrated an MIC90 of 0.25 ~g/ml. The results of this study indicate a high level of susceptibility to mupirocin in S. aureus isolates. Using a break point of 4 ~g/ml, the percentage susceptibility of the 1000 isolates was 99.1%. The nasal and nonnasal isolates were 99.8% and 98.6% susceptible, respectively. Using the Fisher's exact test, these values were not statistically different (P = .089). MRSA isolates demonstrated a susceptibility of 95.9%, which is statistically lower (P = .0001) than
Note
TABLE 1
173
Characterization of 1000 Staphylococcus aureus Isolates Total No. of Isolates
% Total
% Nasal Isolates
% Nonnasal Isolates
1000 414 586 222 778
100 41.4 58.6 22.2 77.8
---23.7 76.3
---21.2 78.8
Total Nasal Nonnasal MRSA MSSA
MRSA, methicillinresistant; and MSSA, methicillinsuscpetible. TABLE 2
All isolates Nasal Nonnasal MRSA MSSA
Comparison of Mupirocin MICs for Nasal and Nonnasal Isolates MICso (~g/ml)
MICgo (p.g/ml)
MIC Ranges (~g/ml)
No. with MIC >4 ~g/ml
% Susceptible
0.25 0.25 0.25 0.25 0.25
0.5 0.5 0.25 0.5 0.25
~0.06->256 ~0.06-16 ~0.06-> 256 40.125->256 ~0.06-2
9 1 8 9 0
99.1 99.8 98.6 95.9 100
the 100% susceptibility of the MSSA isolates. The break point used in this study, 4 ~g/ml, may be conservative for elimination of colonization of the skin surface, because the concentration applied, 20,000 ~g/ml, greatly exceeds this break point; however, currently there are no recognized standards for assessing susceptibility of bacteria to topical agents. To compare the effectiveness of topical agents, it would be helpful to establish distinct break-point criteria, because there are obvious differences in the amount available on a surface following the application of a topical agent when compared with agents that are administered by the oral and intravenous routes. Also, there may be differences in the amount of antibiotic needed to treat skin infections as opposed to clearance of surface nasal colonization. Infecting bacteria lie below the skin surface, and some penetration of the antibiotic is required; however, colonizing bacteria lie on the surface of the skin, and are therefore more likely to be exposed to the maximum concentration of applied antibiotic. Only nine isolates of the 1000 tested had a mupirocin MIC of >4 ~g/ml, and only one of the nine was a nasal isolate. This isolate had an MIC of 16 ~g/ml, and would most likely be eradicated after exposure to the nasal formulation. The eight mupirocin-resistant nonnasal isolates were randomly collected from clinical specimens submitted to SmithKline Beecham Clinical Laboratories, and their history of exposure to mupirocin is unknown. All nine resistant isolates were MRSA. The MIC90 of the nasal isolates was only a single dilution higher than the nonnasal isolates, and
their percent susceptibilities were not statistically different. It appeared that most nasal and nonnasal isolates had similar susceptibility to mupirocin, and there was an even distribution of MRSA between the nasal and nonnasal isolates. This is not unexpected, because the same strains are often found colonizing both the nares and other body sites. The importance of nasal carriage of staphylococci in the epidemiology of hospital staphylococcal infection has long been recognized. Often strains causing infection are indistinguishable from those colonizing the patient's nose. Our studies demonstrate that the susceptibility of staphylococci isolated from the nose or other sites is similar, whether the strains be MRSA or MSSA. Use of mupirocin to eliminate nasal carriage has also been associated with concomitant eradication of carriage on hands (Reagan et al., 1991) and on other body sites (Hill et al., 1988). In addition, there was a significant decrease in the MRSA colonization rate w h e n both nares and w o u n d s were treated with mupirocin (Kauffman et al., 1993). In a review of MRSA in hospitals and long-term care facilities, mupirocin appeared to be one of the most effective topical regimens, and it is safer and more effective than many systemic antibiotics used either alone or in combination with oral agents (Boyce, 1992). The efficacy of mupirocin in eradication of nasal carriage of S. aureus and in the prevention of nosocomial infections has been demonstrated. Recent studies have shown the ability of mupirocin to reduce colonization of intravenous cannulae (Hill et al., 1993) and decrease the incidence of staphylococcal bacteremias in hemodialysis and continuous am-
174
bulatory peritoneal dialysis patients (Boelaert et al., 1993; Perez-Fontan et al., 1993). With an increase in hospital-acquired infections being caused by multiple drug-resistant staphylococci, infection control by means of eradication of
L.J. Utrup et al.
nasal carriage by topical agents will become increasingly important. This study d e m o n s t r a t e d 99% susceptibility to the topical agent mupirocin a m o n g a r a n d o m l y selected collection of 1000 clinical isolates of S. aureus.
REFERENCES Boyce JM (1981) Nosocomial staphylococcal infections. Ann Intern Med 95:241-242. Boyce JM (1989) Methicillin-resistant Staphylococcus aureus, detection, epidemiology, and control measures. Infect Dis Clin N Am 3:901-913. Boyce JM (1992) Methicillin-resistant Staphylococcus aureus in hospitals and long-term care facilities: microbiology, epidemiology, and preventive measures. Infect Control Hosp Epidemiol 13:725-737. Boelaert JR, Landuyt HW, Godard CA, Daneels RF, Schurgers ML, Mattys EG, De Baere YA, Gheyle DW, Gordts BZ, Herwaldt LA (1993) Nasal mupirocin ointment decreases the incidence of Staphylococcus aureus bacteraemias in haemodialysis patients. Nephrol Dial Transplant 8:235-239. Casewell MW, Hill RLR (1986) Elimination of nasal carriage of Staphylococcus aureus with mupirocin ("pseudomonic acid"): a controlled trial. J Antimicrob Chemother 17:365-372. Haley RW, Hightower AW, Khabbaz RF, Thornsberry C, Martone WJ, Allen JR, Hughes JM (1982) The emergence of methicillin-resistant Staphylococcus aureus infections in United States hospitals. Ann Intern Med 97: 297-308. Hill RLR, Duckworth GJ, Casewell MW (1988) Elimination of nasal carriage of methicillin-resistant Staphylococcus aureus with mupirocin during a hospital outbreak. J Antimicrob Chemother 22:377-384. Hill R, Ben Salem R, Casewell MW (1993) Topical calcium mupirocin for the prevention of colonisation of intravascular cannulae: a randomised double-blind controlled trial [abst 1212]. In Abstracts of the 33rd Interscience Conference on Antimicrobial Agents and Chemotherapy, New Orleans, LA. Washington, DC: American Society for Microbiology. Hughes J, Mellows G (1978) On the mode of action of pseudomonic acid: inhibition of protein synthesis in Staphylococcus aureus. J Antibiot 31:330-335. Kauffman CA, Terpenning MS, He X, Zarins LT, Ramsey MA, Jorgensen KA, Sottile WS, Bradley SF (1993) Attempts to eradicate methicillin-resistant Staphylococcus aureus from a long-term-care facility with the use of mupirocin ointment. Am J Med 94:371-378. Mylotte JM, Karuza J, Bentley DW (1992) Methicillin-
resistant Staphylococcus aureus: a questionnaire survey of 75 long-term care facilities in western New York. Infect Control Hosp Epidemiol 13:711-718. National Committee for Clinical Laboratory Standards (NCCLS) (1993) Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically: approved standards M7-A2, 2nd ed. Villanova, PA: NCCLS. Panlilio AL, Culver DH, Gaynes RP, Banerjee S, Henderson TS, Tolson JS, Martone WJ (1992) Methicillinresistant Staphylococcus aureus in U.S. hospitals, 19751991. Infect Control Hosp Epidemiol 13:582-586. Pappa KA (1990) The clinical development of mupirocin. J Am Acad Dermatol 22:873-879. Perez-Fontan M, Garcia-Falcon T, Rosales M, RodreguezCarmona A, Adeva M, Roderiguez-Lozano I, Moncalian J (1993) Treatment of Staphylococcus aureus nasal carriers in continuous ambulatory peritoneal dialysis with rnupirocin: long-term results. Am J Kidney Dis 22: 708-712. Reagan DR, Doebbeling BN, Pfaller MA, Sheetz CT, Houston AK, HoUis RJ, Wenzel RP (1991) Elimination of coincident Staphylococcus aureus nasal and hand carriage with intranasal application of mupirocin calcium ointment. Ann Intern Med 114:101-106. Redhead RJ, Lamb YJ, Rowsell RB (1991) The efficacy of calcium mupirocin in the eradication of nasal Staphylococcus aureus carriage. Br J Clin Prac 45:252-254. Sutherland R, Boon RJ, Griffin KE, Masters PJ, Slocombe B, White AR (1985) Antibacterial activity of mupirocin (pseudomonic acid), a new antibiotic for topical use. Antimicrob Agents Chemother 27:495-498. Thompson RL, Cabezudo I, Wenzel RP (1982) Epidemiology of nosocomial infections caused by methicillinresistant Staphylococcus aureus. Ann Intern Med 97:309317. Ward TT, Stausbaugh LJ (1992) Increasing prevalence of methiciltin resistant Staphylococcus aureus in nursing homes: the Oregon experience. Infect Med 5:46-51. Wenzel RP, Nettleman MD, Jones RN, Pfaller MA (1991) Methicillin-resistant Staphylococcus aureus: implications for the 1990s and effective control measures. Am J Med 91(Suppl 3B):221S-227S.
171
Comparison of Mupirocin Susceptibility of Nasal and Nonnasal Staphylococcus aureus Isolates Linda J. Utrup, Jane E. Finlay, Stephen F. Rittenhouse, and James A. Poupard
Susceptibilities of 414 nasal and 586 nonnasal Staphylococcus aureus isolates, both methiciUin resistant and methicillin susceptible, to the topical antimicrobial agent mupirocin were compared. A susceptibility of 99.1% was observed for the I000
isolates. Nasal and nonnasal isolates showed a similar 90% minimum inhibitory concentration (MIC9o) and statistically equivalent percent susceptibilities.
The incidence of nosocomial staphylococcal infections and outbreaks is increasing worldwide (Boyce, 1989; Haley et al., 1982), and in many countries, including the United States, staphylococci have emerged as major hospital pathogens. The emergence of methicillin-resistant Staphylococcus aureus (MRSA) has created a significant dilemma, because therapeutic choices are limited. Rates of MRSA infections and colonization are increasing in medical center, community hospital, and nursing home patients (Boyce, 1989; Haley et al., 1982; Mylotte et al., 1992; Panlilio et al., 1992; Ward and Stausbaugh, 1992). The control of staphylococcal nosocomial transmission is important because some MRSA strains have the ability to spread rapidly and cause considerable morbidity and mortality (Boyce, 1992). Once MRSA is introduced into a hospital or nursing home, eradication can be difficult (Boyce, 1981).
In long-term care facilities, patients and healthcare workers may carry staphylococci in their nares or on their hands, and some patients become infected after they are colonized (Boyce, 1989; Thompson et al., 1982). Reservoirs such as the nares allow staphylococci to exchange genetic material with drug-resistant organisms or to persist and be exposed to the selective pressures that lead to the development of resistance. Eradication of the nasal carriage state in both patients and health-care workers is important in controlling staphylococcal infections. Topical antimicrobials are particularly beneficial in eliminating staphylococcal nasal carriage, while reducing the possibility of toxicity associated with systemic agents (Wenzel et al., 1991). Mupirocin, the approved generic name for pseudomonic acid A, a fermentation product of Pseudomonas fluorescens NCIB 10586, has the unique mode of action of inhibiting bacterial isoleucyl tRNA synthetase, thus blocking translation and protein synthesis (Hughes and Mellows, 1978). Mupirocin lacks cross resistance with other antibacterial agents and exhibits activity against strains of bacteria that are multiply drug resistant (Pappa, 1990; Sutherland et al., 1985). Mupirocin in a polyethylene glycol base is approved for use in most countries, including the United States, for treatment of impetigo caused by staphylococci and streptococci. A mupirocin nasal formulation in a white, soft paraffin and lanolin ointment is approved in Europe, and is undergoing clinical
From the Department of Microbial Cell Sciences, Clinical Microbiology, SmithKline Beecham Pharmaceuticals, King of Prussia, Pennsylvania, USA. L.J. Utrup is currently employed at a different institution. Data on 230 isolates were previously published in ICAAC Poster 1141, 1992. Address reprint requests to J. Finlay, SmithKline Beecham Pharmaceuticals, PO Box 1539, King'of Prussia, PA 19406, USA. Received 26 July 1994; revised and accepted 6 September 1994. © 1994Elsevier Science Inc. 655 Avenue of the Americas, New York, NY 10010 0732-8893/94/$7.00
172
trials in the United States. Clinical studies have shown that the mupirocin nasal formulation is safe and effective for use against MRSA and ~-lactamase-producing staphylococcal isolates carried in the nares (Casewell and Hill, 1986; Redhead et al., 1991; Wenzel et al., 1991). Because mupirocin has been used in the treatment of impetigo in the United States for a number of years, it was determined that a comparison of susceptibility results for nasal and nonnasal isolates would be useful. The purpose of this study was to compare the mupirocin susceptibilities of recent nasal and nonnasal isolates of S. aureus. A total of 1000 S. aureus isolates were included in the study: 414 were nasal and 586 were nonnasal. All isolates were identified as S. aureus, and most were derived from individual patients. Of the nasal isolates, 246 were obtained from a multicenter, US clinical study evaluating the safety, tolerance, and efficacy of calcium mupirocin, and 57 isolates were obtained from a Canadian clinical study evaluating the efficacy of calcium mupirocin in eradicating the carrier state of S. aureus. Only pretreatment isolates from the clinical studies were included in this evaluation. Dr. Stephen Brecher of the Boston Veterans Administration Hospital, Boston, Massachusetts, provided 94 nasal isolates from his collection, most of which were MRSA. Another nine nasal isolates were collected from L. Steele Moore of the Medical Center of Delaware, Wilmington, Delaware, and eight nasal isolates were obtained from SmithKline Beecham Clinical Laboratories (Philadelphia, PA, USA). Of the 586 nonnasal S. aureus isolates, 112 were provided by Dr. Julio Ramirez of the University of Louisville, Louisville, Kentucky, and the remainder were obtained from SmithKline Beecham Clinical Laboratories. Previous exposure of the isolates to mupirocin is unknown; however, all isolates were collected since the introduction of mupirocin in the United States. Organisms were isolated on 5% sheep blood agar plates (Becton Dickinson Microbiology Systems, Cockeysville, MD, USA) and stored at -70°C in trypticase soy broth with 20% glycerol (Becton Dickinson Microbiology Systems). Staphylococcus aureus isolates were identified by the Staph Ident System (bioMerieux Vitek) along with tube coagulase (Becton Dickinson Microbiology Systems) and catalase (Mallinckrodt AR, Paris, KY, USA) tests. Methicillin resistance was determined by using MRSA Screen Agar (Becton Dickinson Microbiology Systems) according to the recommended procedure of the manufacturer. Minimum inhibitory concentration (MIC) determinations were performed according to National Committee for Clinical Laboratory Standards recommended methods (NCCLS, 1993). Microtiter MIC
L.J. Utrup et al.
trays were prepared in-house employing an automatic dispensing system (Quick Spense lie; Sandy Spring Instrument Company, Germantown, MD, USA). Trays were stored at -70°C. All wells contained 0.05 ml of antibiotic solution in doubling dilutions. Mupirocin (SmithKline Beecham) was dissolved in water. The MIC trays were thawed and inoculated with an equal volume, 0.05 ml, of a 1:100 dilution of a 0.5 McFarland adjusted culture so that the final organism concentration in the microtiter wells was 5 x 10s colony-forming units/ml. The McFarland adjusted culture was prepared from four to five colonies grown at 35°-37°C for 18-24 h on 5% sheep blood agar plates (Becton Dickinson Microbiology Systems). Microtiter trays were incubated for 18-24 h at 35°-37°C. The MICs were read as the lowest concentration of mupirocin required to inhibit visible growth of the organisms. A break point of 4 4 ~g/ml was used. This break point was determined following an evaluation of the interpretive criteria for mupirocin susceptibility testing using a large number of staphylococci, including strains with high MICs. Major errors were minimized by using an MIC break point of 4 4 ~g/ml and a 5-~g disk break point of ~18 mm. These break points were used in the clinical trials from which most of the isolates were obtained. The distribution of the 1000 isolates, including the number of nasal and nonnasal isolates, and the methicillin susceptibility of the isolates are listed in Table 1: 41% of the isolates were nasal, 59% were nonnasal, and 22% were MRSA. Table 1 also demonstrates the characteristics of the nasal and nonnasal isolates. There was an approximately even distribution of MRSA b e t w e e n the two groups of isolates. A comparison of the 1000 S. aureus isolates by MICs0, MIC90, MIC ranges, the number of organisms with MICs >4 ~g/ml, and the percent susceptible organisms are presented in Table 2. The MICs0 for each group of isolates was 0.25 ~g/ml. The MICg0 for the 1000 isolates and for the nasal isolates was 0.5 ~g/ml, whereas the nonnasal isolate MICg0 was 0.25 ~g/ml. Isolates that were found to be MRSA had an MICg0 of 0.5 ~g/ml, whereas methicillin susceptible S. aureus (MSSA) isolates demonstrated an MIC90 of 0.25 ~g/ml. The results of this study indicate a high level of susceptibility to mupirocin in S. aureus isolates. Using a break point of 4 ~g/ml, the percentage susceptibility of the 1000 isolates was 99.1%. The nasal and nonnasal isolates were 99.8% and 98.6% susceptible, respectively. Using the Fisher's exact test, these values were not statistically different (P = .089). MRSA isolates demonstrated a susceptibility of 95.9%, which is statistically lower (P = .0001) than
Note
TABLE 1
173
Characterization of 1000 Staphylococcus aureus Isolates Total No. of Isolates
% Total
% Nasal Isolates
% Nonnasal Isolates
1000 414 586 222 778
100 41.4 58.6 22.2 77.8
---23.7 76.3
---21.2 78.8
Total Nasal Nonnasal MRSA MSSA
MRSA, methicillinresistant; and MSSA, methicillinsuscpetible. TABLE 2
All isolates Nasal Nonnasal MRSA MSSA
Comparison of Mupirocin MICs for Nasal and Nonnasal Isolates MICso (~g/ml)
MICgo (p.g/ml)
MIC Ranges (~g/ml)
No. with MIC >4 ~g/ml
% Susceptible
0.25 0.25 0.25 0.25 0.25
0.5 0.5 0.25 0.5 0.25
~0.06->256 ~0.06-16 ~0.06-> 256 40.125->256 ~0.06-2
9 1 8 9 0
99.1 99.8 98.6 95.9 100
the 100% susceptibility of the MSSA isolates. The break point used in this study, 4 ~g/ml, may be conservative for elimination of colonization of the skin surface, because the concentration applied, 20,000 ~g/ml, greatly exceeds this break point; however, currently there are no recognized standards for assessing susceptibility of bacteria to topical agents. To compare the effectiveness of topical agents, it would be helpful to establish distinct break-point criteria, because there are obvious differences in the amount available on a surface following the application of a topical agent when compared with agents that are administered by the oral and intravenous routes. Also, there may be differences in the amount of antibiotic needed to treat skin infections as opposed to clearance of surface nasal colonization. Infecting bacteria lie below the skin surface, and some penetration of the antibiotic is required; however, colonizing bacteria lie on the surface of the skin, and are therefore more likely to be exposed to the maximum concentration of applied antibiotic. Only nine isolates of the 1000 tested had a mupirocin MIC of >4 ~g/ml, and only one of the nine was a nasal isolate. This isolate had an MIC of 16 ~g/ml, and would most likely be eradicated after exposure to the nasal formulation. The eight mupirocin-resistant nonnasal isolates were randomly collected from clinical specimens submitted to SmithKline Beecham Clinical Laboratories, and their history of exposure to mupirocin is unknown. All nine resistant isolates were MRSA. The MIC90 of the nasal isolates was only a single dilution higher than the nonnasal isolates, and
their percent susceptibilities were not statistically different. It appeared that most nasal and nonnasal isolates had similar susceptibility to mupirocin, and there was an even distribution of MRSA between the nasal and nonnasal isolates. This is not unexpected, because the same strains are often found colonizing both the nares and other body sites. The importance of nasal carriage of staphylococci in the epidemiology of hospital staphylococcal infection has long been recognized. Often strains causing infection are indistinguishable from those colonizing the patient's nose. Our studies demonstrate that the susceptibility of staphylococci isolated from the nose or other sites is similar, whether the strains be MRSA or MSSA. Use of mupirocin to eliminate nasal carriage has also been associated with concomitant eradication of carriage on hands (Reagan et al., 1991) and on other body sites (Hill et al., 1988). In addition, there was a significant decrease in the MRSA colonization rate w h e n both nares and w o u n d s were treated with mupirocin (Kauffman et al., 1993). In a review of MRSA in hospitals and long-term care facilities, mupirocin appeared to be one of the most effective topical regimens, and it is safer and more effective than many systemic antibiotics used either alone or in combination with oral agents (Boyce, 1992). The efficacy of mupirocin in eradication of nasal carriage of S. aureus and in the prevention of nosocomial infections has been demonstrated. Recent studies have shown the ability of mupirocin to reduce colonization of intravenous cannulae (Hill et al., 1993) and decrease the incidence of staphylococcal bacteremias in hemodialysis and continuous am-
174
bulatory peritoneal dialysis patients (Boelaert et al., 1993; Perez-Fontan et al., 1993). With an increase in hospital-acquired infections being caused by multiple drug-resistant staphylococci, infection control by means of eradication of
L.J. Utrup et al.
nasal carriage by topical agents will become increasingly important. This study d e m o n s t r a t e d 99% susceptibility to the topical agent mupirocin a m o n g a r a n d o m l y selected collection of 1000 clinical isolates of S. aureus.
REFERENCES Boyce JM (1981) Nosocomial staphylococcal infections. Ann Intern Med 95:241-242. Boyce JM (1989) Methicillin-resistant Staphylococcus aureus, detection, epidemiology, and control measures. Infect Dis Clin N Am 3:901-913. Boyce JM (1992) Methicillin-resistant Staphylococcus aureus in hospitals and long-term care facilities: microbiology, epidemiology, and preventive measures. Infect Control Hosp Epidemiol 13:725-737. Boelaert JR, Landuyt HW, Godard CA, Daneels RF, Schurgers ML, Mattys EG, De Baere YA, Gheyle DW, Gordts BZ, Herwaldt LA (1993) Nasal mupirocin ointment decreases the incidence of Staphylococcus aureus bacteraemias in haemodialysis patients. Nephrol Dial Transplant 8:235-239. Casewell MW, Hill RLR (1986) Elimination of nasal carriage of Staphylococcus aureus with mupirocin ("pseudomonic acid"): a controlled trial. J Antimicrob Chemother 17:365-372. Haley RW, Hightower AW, Khabbaz RF, Thornsberry C, Martone WJ, Allen JR, Hughes JM (1982) The emergence of methicillin-resistant Staphylococcus aureus infections in United States hospitals. Ann Intern Med 97: 297-308. Hill RLR, Duckworth GJ, Casewell MW (1988) Elimination of nasal carriage of methicillin-resistant Staphylococcus aureus with mupirocin during a hospital outbreak. J Antimicrob Chemother 22:377-384. Hill R, Ben Salem R, Casewell MW (1993) Topical calcium mupirocin for the prevention of colonisation of intravascular cannulae: a randomised double-blind controlled trial [abst 1212]. In Abstracts of the 33rd Interscience Conference on Antimicrobial Agents and Chemotherapy, New Orleans, LA. Washington, DC: American Society for Microbiology. Hughes J, Mellows G (1978) On the mode of action of pseudomonic acid: inhibition of protein synthesis in Staphylococcus aureus. J Antibiot 31:330-335. Kauffman CA, Terpenning MS, He X, Zarins LT, Ramsey MA, Jorgensen KA, Sottile WS, Bradley SF (1993) Attempts to eradicate methicillin-resistant Staphylococcus aureus from a long-term-care facility with the use of mupirocin ointment. Am J Med 94:371-378. Mylotte JM, Karuza J, Bentley DW (1992) Methicillin-
resistant Staphylococcus aureus: a questionnaire survey of 75 long-term care facilities in western New York. Infect Control Hosp Epidemiol 13:711-718. National Committee for Clinical Laboratory Standards (NCCLS) (1993) Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically: approved standards M7-A2, 2nd ed. Villanova, PA: NCCLS. Panlilio AL, Culver DH, Gaynes RP, Banerjee S, Henderson TS, Tolson JS, Martone WJ (1992) Methicillinresistant Staphylococcus aureus in U.S. hospitals, 19751991. Infect Control Hosp Epidemiol 13:582-586. Pappa KA (1990) The clinical development of mupirocin. J Am Acad Dermatol 22:873-879. Perez-Fontan M, Garcia-Falcon T, Rosales M, RodreguezCarmona A, Adeva M, Roderiguez-Lozano I, Moncalian J (1993) Treatment of Staphylococcus aureus nasal carriers in continuous ambulatory peritoneal dialysis with rnupirocin: long-term results. Am J Kidney Dis 22: 708-712. Reagan DR, Doebbeling BN, Pfaller MA, Sheetz CT, Houston AK, HoUis RJ, Wenzel RP (1991) Elimination of coincident Staphylococcus aureus nasal and hand carriage with intranasal application of mupirocin calcium ointment. Ann Intern Med 114:101-106. Redhead RJ, Lamb YJ, Rowsell RB (1991) The efficacy of calcium mupirocin in the eradication of nasal Staphylococcus aureus carriage. Br J Clin Prac 45:252-254. Sutherland R, Boon RJ, Griffin KE, Masters PJ, Slocombe B, White AR (1985) Antibacterial activity of mupirocin (pseudomonic acid), a new antibiotic for topical use. Antimicrob Agents Chemother 27:495-498. Thompson RL, Cabezudo I, Wenzel RP (1982) Epidemiology of nosocomial infections caused by methicillinresistant Staphylococcus aureus. Ann Intern Med 97:309317. Ward TT, Stausbaugh LJ (1992) Increasing prevalence of methiciltin resistant Staphylococcus aureus in nursing homes: the Oregon experience. Infect Med 5:46-51. Wenzel RP, Nettleman MD, Jones RN, Pfaller MA (1991) Methicillin-resistant Staphylococcus aureus: implications for the 1990s and effective control measures. Am J Med 91(Suppl 3B):221S-227S.