P2056 Determination of the minimum inhibitory concentration and mutant prevention concentration (MPC) of tigecycline against clinical isolates of Streptococcus pneumoniae; impact of media on MPC results

Pharmacodynamics and untoward effects of antibacterials drug’s activity to be aware of which interpretive breakpoints were used in the analysis, and to bear in mind that there can be significant differences when using EUCAST instead of FDA breakpoints. P2055 The killing of E. coli by tigecycline using the minimum inhibitory and mutant prevention drug concentration J. Blondeau, S. Borsos (Saskatoon, CA) Objective: Tigecycline (TIG) is a new antibacterial agent with low minimum inhibitory concentration (MIC) and low mutant prevention concentration (MPC) against clinical isolates of E. coli. We were interested in determining the rate and extent of killing by Tig using MIC and MPC drug concentrations against high density inocula of E. coli. Methods: For MIC testing, 105 cfu/mL of E. coli were exposed to doubling drug concentration of Tig in Mueller-Hinton (MH) broth and following incubation under ambient conditions, the lowest concentration preventing growth was the MIC. For MPC testing, 109 CFUs were exposed to doubling dilutions of TIG on drug containing MH agar plates and following incubation for 24−48 hours, the lowest drug concentration preventing growth was the MPC. For kill experiments, 106 cfu/mL and 107 cfu/mL were exposed to the measured MIC and MPC drug concentrations and the reduction in viable cells (log10 and % kill) were recorded at 30 min, 1, 2, 3, 4, 6, 12 and 24 hours. All experiments were conducted in triplicate. Results: For 2 clinical E. coli isolates, MIC values were 0.063 mg/L and MPC values were 1 mg/L. When 106 or 107 cfu/mL were exposed to the MIC drug concentrate, positive growth was seen for the first 6 hours of drug exposure; at 106 cfu/mL, a 1.06 log10 reduction (13% kill) was seen by 12 hours and 1.64 log10 reduction (97.7% kill by 24 hours). Exposure of 106 cfu/mL to the MPC drug concentration yielded a 0.16 to 0.42 log10 reduction (31−62% kill) by 30 min to 4 hours and this increased to a 1.16–1.86 log10 reduction (93−98% kill by 12−24 hours). Exposure of 107 cfu/mL to the MPC drug concentration yielded similar results: 0.13–0.31 log10 reduction (26−51% kill) by 1−4 hours and 0.71–1.14 log10 reduction (81−99% kill) by 12 hours. Conclusion: TIG MIC and MPC values were low against E. coli – 0.031 and 1 mg/L respectively. Killing using MIC drug concentrations against higher density inocula was slow and incomplete. Killing was faster and more complete following exposure to MPC drug concentrations – 51−62% by 4 hours and 81−99% by 12 hours. The low MIC and MPC values suggest that TIG is less likely to select for resistant E. coli and dosing to achieve MPC results in the efficient killing of high density bacterial burdens. P2056 Determination of the minimum inhibitory concentration and mutant prevention concentration (MPC) of tigecycline against clinical isolates of Streptococcus pneumoniae; impact of media on MPC results C. Hesje, S. Borsos, J. Blondeau (Saskatoon, CA) Objective: Tigecycline is the first of a new class of compound – glycylcyclines – with reported potent in vitro activity against penicillin-susceptible and multi-drug resistant strains of Streptococcus pneumoniae (SP). The mutant prevention concentration (MPC) defines the antimicrobial drug concentration threshold that blocks the growth of resistant bacterial sub-populations that may be present in high density bacterial populations such as those present in infection. We measured the MIC and MPC values for tigecycline against clinical isolates of SP and compared MPC results on blood agar versus solidifying Todd-Hewett broth (THB). Methods: For MIC testing, the recommended Clinical and Laboratory Standards Institute procedure was followed utilising 105 cfu/mL tested against doubling drug dilutions in THB with incubation at 35−37 degrees Celsius in 5% CO2 for 18−24 hours. For MPC testing, 109 CFUs were added to drug containing agar plates: 1) tryptic soy agar containing 5% sheep red blood cells (BA), 2) THB solidified with 1.5% agar. Inoculated plates were incubated as described for 24−48 hours and screened for

S593 growth. The lowest drug concentration preventing growth was the MIC or MPC depending on method. Results: For 140 SP isolates (121/86.4% penicillin susceptible; 19/13.6% penicillin non-susceptible), MIC50 , MIC90 and MIC range values were 0.016 mg/L, 0.016 mg/L and
0.008–0.31 mg/L respectively and these MIC values were not influenced by SP susceptibility or resistance to penicillin nor for isolates recovered from blood or respiratory tract specimens. MPC50, MPC90 and MPC values were 8 mg/L, 16 mg/L and 1−16 mg/L when tested on BA. MPC testing on solidified THB yielded MPC50, MPC90 and MPC range values of 0.063 mg/L, 0.12 mg/L and 0.063–0.25 mg/L. Conclusion: Tigecycline was highly active in vitro against SP with MICs
0.031 mg/L and not different for strains of SP resistant to penicillin. MPC values ranged from 1−16 mg/L on BA but we were substantially lower on solidified THB – 0.063–0.25 mg/L. This data suggests that SP testing of tigecycline on BA yields falsely elevated values. Solidified THB appears to be a more suitable media for SP MPC testing against tigecycline. Tigecycline appears to have a low propensity to select for resistance.

Pharmacodynamics and untoward effects of antibacterials P2057 Susceptibility of various b-lactams to the inoculum effect V.H. Tam, K.R. Ledesma, K.T. Chang, T.Y. Wang, J.P. Quinn (Houston, Chicago, US) Objective: Escherichia coli (EC) is part of the human gastrointestinal flora and a common pathogen implicated in intra-abdominal infections such as perforated appendicitis and peritonitis. A heavy bacterial burden is anticipated in this type of infection and the clinical utility of the b-lactams may be limited by the inoculum effect. We compared the bactericidal activity of various b-lactam sub-classes against a standard and heavy inoculum of EC. Methods: A wild-type (EC ATCC 25922) and a clinical ESBLproducing (TEM-26) strain were used. Time-kill studies were performed using approximately 105 and 108 cfu/mL at baseline. A clinically achievable concentration range of piperacillin/tazobactam (PIPT), ceftriaxone (CRO) and ertapenem (ERT) were used, and the drug concentrations were normalised to multiples of MIC. Serial samples were obtained in duplicate over 24 hours; viable bacterial burden was determined by quantitative culture to examine the impact of a higher inoculum on the bactericidal activity of various b-lactams. Results: MIC of the wild type strain to PIPT, CRO and ERT were 2/4, 0.125 and 0.008 mg/L; for TEM-26 the respective MIC values were 16/4, 64 and 0.06 mg/L. All 3 b-lactams demonstrated significant killing with the standard inoculum (except for CRO against TEM-26). However, with the higher inoculum, the activity of PIPT was drastically reduced in both strains. CRO remained reasonably bactericidal against the wild type strain only (at 16×MIC). ERT was the least affected by the inoculum effect in both strains; bactericidal activity was retained with concentrations 4−8×MIC. Conclusion: Our results suggest that different b-lactam sub-classes have a distinct killing profile against a dense EC population. ERT appeared to be the least susceptible to the inoculum effect, which might be more efficacious than other non-carbapenem b-lactams in the treatment of intra-abdominal infections. Comparative in-vivo/clinical investigations are warranted to validate our findings. P2058 Extracellular and intracellular activities of quinupristindalfopristin (Synercid) against Staphylococcus aureus, with different resistant phenotypes (MSSA, MRSA, VISA) P. Baudoux, Y. Glupczynski, P. Tulkens, F. Van Bambeke (Brussels, BE) Objectives: S. aureus survives and thrives in mild acidic pH environments, such as found intracellularly in phagolysosomes. Synercid, a semi-synthetic streptogramin antibiotic composed of quinupristin