Bioactivity of imipenem as a function of medium, time, and temperature

Antimicrobial susceptibility among clinical Nocardia species identified by multilocus sequence analysis

Antimicrobial susceptibility patterns of 112 clinical isolates, 28 type strains, and 9 reference strains of Nocardia were determined using the Sensititre Rapmyco microdilution panel (Thermo Fisher, Inc.). Isolates were identified by highly discriminatory multilocus sequence analysis and were chosen to represent the diversity of species recovered from clinical specimens in Ontario, Canada. Susceptibility to the most commonly used drug, trimethoprim-sulfamethoxazole, was observed in 97% of isolates. Linezolid and amikacin were also highly effective; 100% and 99% of all isolates demonstrated a susceptible phenotype. For the remaining antimicrobials, resistance was species specific with isolates of Nocardia otitidiscaviarum, N. brasiliensis, N. abscessus complex, N. nova complex, N. transvalensis complex, N. farcinica, and N. cyriacigeorgica displaying the traditional characteristic drug pattern types. In addition, the antimicrobial susceptibility profiles of a variety of rarely encountered species isolated from clinical specimens are reported for the first time and were categorized into four additional drug pattern types. Finally, MICs for the control strains N. nova ATCC BAA-2227, N. asteroides ATCC 19247(T), and N. farcinica ATCC 23826 were robustly determined to demonstrate method reproducibility and suitability of the commercial Sensititre Rapmyco panel for antimicrobial susceptibility testing of Nocardia spp. isolated from clinical specimens. The reported values will facilitate quality control and standardization among laboratories.

Antimicrobial susceptibility testing of carbapenems: multicenter validity testing and accuracy levels of five antimicrobial test methods for detecting resistance in Enterobacteriaceae and Pseudomonas aeruginosa isolates

From January 1996 to May 1999, Project ICARE (Intensive Care Antimicrobial Resistance Epidemiology) received 448 nonduplicate clinical isolates of Enterobacteriaceae and Pseudomonas aeruginosa that were reported to be imipenem intermediate or resistant. However, broth microdilution (BMD) confirmatory testing at the Project ICARE central laboratory confirmed this result in only 11 of 123 (8.9%) Enterobacteriaceae isolates and 241 of 325 (74.2%) P. aeruginosa isolates. To investigate this overdetection of imipenem resistance, we tested 204 selected isolates from the Project ICARE collection plus five imipenem-resistant challenge strains at the Centers for Disease Control and Prevention against imipenem and meropenem by agar dilution, disk diffusion, Etest (AB BIODISK North America, Inc., Piscataway, N.J.), two MicroScan WalkAway conventional panels (Neg MIC Plus 3 and Neg Urine Combo 3) (Dade MicroScan, Inc., West Sacramento, Calif.), and two Vitek cards (GNS-116 containing meropenem and GNS-F7 containing imipenem) (bioMérieux Vitek, Inc., Durham, N.C.). The results of each test method were compared to the results of BMD testing using in-house-prepared panels. Seven imipenem-resistant and five meropenem-resistant isolates of Enterobacteriaceae and 43 imipenem-resistant and 21 meropenem-resistant isolates of P. aeruginosa were identified by BMD. For Enterobacteriaceae, the imipenem and meropenem test methods produced low numbers of very major and major errors. All test systems in the study produced low numbers of very major and major errors when P. aeruginosa was tested against imipenem and meropenem, except for Vitek testing (major error rate for imipenem, 20%). Further testing conducted in 11 of the participating ICARE hospital laboratories failed to pinpoint the factors responsible for the initial overdetection of imipenem resistance. However, this study demonstrated that carbapenem testing difficulties do exist and that laboratories should consider using a second, independent antimicrobial susceptibility testing method to validate carbapenem-intermediate and -resistant results.

Effect of zinc concentration in Mueller-Hinton agar on susceptibility of Pseudomonas aeruginosa to imipenem

The susceptibility of Pseudomonas aeruginosa to imipenem has been shown to vary according to zinc concentration in the media. MICs of imipenem for 68 unique clinical isolates of P. aeruginosa were determined in media supplemented with zinc at concentrations between 0.5 and 6.0 micrograms/ml. In agar containing up to 3 micrograms of zinc/ml, 75 to 82% of the strains were susceptible to imipenem at an MIC of < or = 4 micrograms/ml. In agar supplemented to contain 6 micrograms of zinc/ml, however, only 40% of the strains were susceptible to imipenem. Manufacturers should ensure that the concentration of zinc in commercial media is below 3 micrograms/ml to avoid false classification of isolates as resistant to imipenem.

Beta-lactam stability in frozen microdilution PASCO MIC panels using strains with known resistance mechanisms as biosensors

The stability of amoxicillin/clavulanate, piperacillin/tazobactam, cefepime, imipenem, and meropenem in PASCO (PASCO System, DIFCO Laboratories, Detroit, MI, USA) frozen microdilution susceptibility panels stored for 16 weeks at -20 degrees C and -70 degrees C was evaluated. The increase in MIC values for the five American-Type Culture Collection (ATCC) quality control strains for susceptibility testing recommended by the National Committee for Clinical Laboratory Standards (NCCLS) and for 13 strains with different well-characterized resistance mechanisms was indicative of bioactivity deterioration. The overall agreement (+/- 1 twofold dilution) at purchase between the MIC values of PASCO frozen microdilution susceptibility panels and the standard agar dilution method was 97.7%. Minimum inhibitory concentration values for the associations of amoxicillin/clavulanate and piperacillin/tazobactam remained unchanged for the study period at -70 degrees C. In contrast, a carbapenem bioactivity decrease was detected only with strains having well-characterized resistance mechanisms from the 12th week onwards. At -20 degrees C, antibiotic deterioration with these latter strains was observed earlier than with ATCC strains: the activity of meropenem and imipenem remained unchanged only for the first 2 weeks, while a loss of activity was detected for amoxicillin/clavulanate and piperacillin/tazobactam at the 7th and 10th week, respectively. Cefepime was highly stable both at -20 degrees C and -70 degrees C. Strains with well-characterized resistance mechanisms should be used in routine quality control studies of antibiotic stability for susceptibility testing panels during the storage period.

Discrepancies in susceptibility test results for imipenem employing different in vitro test methods and DIN 58,940 breakpoints

During the first half of 1993, bacteria that were isolated from clinical materials and found to have intermediate susceptibility by an agar dilution breakpoint method were collected in a large service laboratory in Germany. All of these isolates were gram-negative bacteria. They were re-tested employing full-scale agar dilution, broth microdilution, E-test and agar diffusion procedures. The results obtained indicated that 76.9% of the isolates were actually susceptible upon re-testing with a reference agar dilution technique. The reason for the discrepant results remained largely unclear. There was a high correlation between agar dilution and E-test results while the agreement with broth microdilution and agar diffusion was less satisfactory. It is suggested that the breakpoint between susceptible and intermediate categories currently recommended by DIN 58,940 (standard set by Deutsches Institut für Normung e.V.) be raised to reduce erroneous interpretations of minimum inhibitory concentrations.

Imipenem stability in a predried susceptibility panel

We performed a 15-month study using 11 clinical strains and 1 control strain (ATCC 27853) of Pseudomonas aeruginosa to determine whether changes in the manufacturing process of Sensititre predried panels result in a reliable test of susceptibility to imipenem. MIC and breakpoint susceptibility results remained stable during the manufacturer's recommended shelf life of 18 months and compared well with standard agar disk diffusion and broth macrodilution results. Imipenem concentrations measured by high-pressure liquid chromatography were acceptable through 15 months but declined in the breakpoint panels by approximately 50% at 18 months. Between 9 months and panel expiration, 13 of 141 (9%) of the MIC panel packages had moisture entry, as indicated by pink desiccants, with a resultant loss of imipenem activity of 32 to 100%. It appears that the new manufacturing process produces MIC panels that are reliable for imipenem susceptibility testing until the labeled expiration date, provided that packages containing pink desiccants are not used.

Susceptibility testing of Listeria monocytogenes. A reassessment of bactericidal activity as a predictor for clinical outcome

In vitro susceptibility testing of Listeria monocytogenes most often reveals both ampicillin and penicillin as inhibitory as opposed to bactericidal with activity comparable to chloramphenicol and tetracycline. Yet, the former two penicillins are more effective for Listeria meningitis than are the latter agents. Accordingly, we reassessed the bactericidal activity of agents used in listeriosis in order to determine in vitro methodology that would be more predictive of clinical outcome. We found that bactericidal activity for greater than 48 hr by either minimum inhibitory-minimum bactericidal concentration (MIC-MBC) testing or time-kill kinetic studies was the best predictor of clinical efficacy. This correlation may be due to Listeria being a slow-growing microorganism. In addition to ampicillin and penicillin, we found trimethoprim-sulfamethoxazole, vancomycin, and imipenem to exhibit bactericidal activity for 48 hr. For the first two agents, this is in agreement with the results of clinical experience.

False resistance to imipenem with a microdilution susceptibility testing system

Routine monitoring of antibiotic resistance at Children's Hospital, Boston, detected a dramatic increase in the prevalence of imipenem-resistant strains of Pseudomonas aeruginosa. Further studies documented that false resistance to imipenem was due, in part, to the loss of imipenem potency in customized MIC microdilution trays supplied by Sensititre Ltd. (West Sussex, United Kingdom). Recognition of the problem was delayed by use of the quality control standard recommended by the manufacturer, which were higher and broader than those suggested by the National Committee for Clinical Laboratory Standards.

Pseudoresistance of Pseudomonas aeruginosa resulting from degradation of imipenem in an automated susceptibility testing system with predried panels

During the use of a single lot of custom breakpoint panels (Sensititre; Radiometer America Inc., Westlake, Ohio), imipenem susceptibility declined from 70 to 44% for clinical isolates of Pseudomonas aeruginosa. With a new lot, susceptibility increased to 73%. Subsequent evaluations with P. aeruginosa ATCC 27853 revealed a similar susceptibility pattern and an increase in the MIC of imipenem when determined in panels with increasing ages. Imipenem concentrations were determined by high-performance liquid chromatography by using 11 different lots of MIC and breakpoint panels (139 to 893 days of age). The amount of imipenem remaining declined from 94.4 to 13.8% (r = 0.9225) over the age range of the panels. These data suggest that imipenem in Sensititre MIC and breakpoint panels degrades over time and that the decrease in imipenem may be largely responsible for the decline in P. aeruginosa susceptibility.

Erythromycin bioactivity is stable in ophthalmic ointment used for prophylaxis of neonatal gonococcal conjunctivitis

Erythromycin ophthalmic ointment (E. Fougera & Co., Melville, N.Y.) and erythromycin gluceptate standards prepared in ointment base were stored at room temperature and heated at temperatures up to 45 degrees C for as long as 6 h before being assayed for bioactivity. We were unable to detect any significant loss of antibiotic bioactivity.

Imipenem/cilastatin. A review of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Imipenem is the first available semisynthetic thienamycin and is administered intravenously in combination with cilastatin, a renal dipeptidase inhibitor that increases urinary excretion of active drug. In vitro studies have demonstrated that imipenem has an extremely wide spectrum of antibacterial activity against Gram-negative and Gram-positive aerobic and anaerobic bacteria, even against many multiresistant strains of bacteria. It is very potent against species which elaborate beta-lactamases. Imipenem in combination with equal doses of cilastatin has been shown to be generally well tolerated and an effective antimicrobial for the treatment of infections of various body systems. It is likely to be most valuable as empirical treatment of mixed aerobic and anaerobic infections, bacteraemia in non-neutropenic patients and serious hospital-acquired infections.

Comparative in vitro activities of ampicillin, BMY 28142, and imipenem against Mycobacterium avium complex

The in vitro activity of ampicillin, BMY 28142, and imipenem was evaluated against 21 clinical isolates of Mycobacterium avium complex by both a broth and an agar dilution method. The MIC90 by broth dilution for ampicillin, BMY 28142, and imipenem was 16 micrograms/ml, 8 micrograms/ml, and greater than 32 micrograms/ml, respectively. The MIC90 by agar dilution for ampicillin and BMY 28142 was 16 micrograms/ml.

Imipenem susceptibility testing with a commercially prepared dry-format microdilution tray

MICs of imipenem, concurrently generated in commercially prepared microdilution trays containing predried antibiotic dilutions (Sensititre), and in a standard agar dilution assay (as recommended by the National Committee for Clinical Laboratory Standards, Villanova, Pa.), were within +/- 1 twofold dilution for 94% of 226 bacterial isolates. Imipenem biological activity remained stable over 5 months of tray storage at room temperature against Pseudomonas aeruginosa ATCC 27853. Activity of imipenem was shown by microdilution testing with 890 clinical isolates to be high, with only 4% of isolates having MICs of greater than or equal to 16 micrograms/ml (in vitro resistance).

Degradation kinetics of imipenem in normal saline and in human serum

The stability of imipenem in 0.9% sodium chloride and in human serum was measured at controlled temperatures over time. The degradation was characterized by a first-order process and was consistent with an apparent hydrolysis reaction.

Stability of imipenem in Mueller-Hinton agar stored at 4 degrees C

The purpose of the present study was to measure the stability of imipenem in Mueller-Hinton agar stored at 4 degrees C over time. MICs for Staphylococcus aureus ATCC 25923, Streptococcus faecalis ATCC 29212, Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853 were determined in triplicate daily for up to 15 days. The calculated mean time to observe a shift of one dilution in MIC endpoints was 4.33 +/- 1.25 days. For routine work, imipenem agar dilution plates should be prepared within 48 to 72 h of the test.

Review of the in vitro spectrum of activity of imipenem

Imipenem (N-formimidoyl thienamycin, MK0787), a new carbapenem was found to have the widest antimicrobial activity of currently available beta-lactam drugs. Enterobacteriaceae had minimal inhibitory concentrations of imipenem of 8.0 micrograms/ml or less for 99.8 percent of clinical isolates. Only rare strains of Enterobacter species and Proteus mirabilis have higher imipenem minimal inhibitory concentration results. Hemophilus and Neisseria species were inhibited, but minimal inhibitory concentrations of imipenem were higher than those reported for third-generation cephalosporins. Only Pseudomonas maltophilia and Pseudomonas cepacia strains were imipenem resistant (MIC50 greater than 32 micrograms/ml) among the commonly isolated non-enteric gram-negative bacilli. All anaerobes were found susceptible to imipenem with the exception of some strains of Clostridium difficile. Staphylococcus species and non-enterococcal streptococci were very susceptible to imipenem. Streptococcus faecalis had higher minimal inhibitory concentrations of imipenem (MIC90 3.1 micrograms/ml) and S. faecium strains were frankly resistant. Methicillin-resistant S. aureus isolates had a MIC90 of 27.2 micrograms imipenem/ml. Imipenem was generally bactericidal except for marked minimal inhibitory and minimal bactericidal concentration differences with enterococci, Listeria, methicillin-resistant staphylococci, and some P. aeruginosa strains. The minimal inhibitory and minimal bactericidal concentrations of imipenem were not significantly influenced by organism inoculum size, probably because of its beta-lactamase stability to nearly all commonly encountered bacterial enzymes. Imipenem was found to be an excellent inhibitor of beta-lactamases and a potent enzyme inducer. The induction characteristic seems responsible for the antagonistic interactions of imipenem with some enzyme-labile beta-lactams in combination. Imipenem had limited stability in some in vitro susceptibility test systems. The 10 micrograms disk test or dry-form broth micro-dilution systems were preferred, applying the interpretive criteria from the National Committee for Clinical Laboratory Standards (M2-A3). Imipenem-resistant strains were rarely found in clinical practice and bacteria resistant to newer beta-lactams and aminoglycosides were generally very susceptible to this new carbapenem.

Imipenem (N-formimidoyl thienamycin): in vitro antimicrobial activity and beta-lactamase stability

In vitro studies with imipenem (N-formimidoyl thienamycin or MK0787) were performed with 8481 clinical isolates in three separate medical centers. More extensive comparative studies were also performed with 605 representative isolates, comparing imipenem to six other beta-lactams. Although the newer beta-lactams were often more active against susceptible species, imipenem demonstrated the broadest spectrum of antibacterial activity, with MIC 90s less than or equal to 4.0 micrograms/ml for all species tested except Pseudomonas maltophilia and P. cepacia. Imipenem was very active against all streptococci and staphylococci, in contrast to the third-generation cephalosporins. There was no evidence of cross-resistance between imipenem and the cephalosporins or penicillins. Resistance to hydrolysis by seven beta-lactamase preparations was documented for imipenem, cefotaxime, and moxalactam. Like many other beta-lactams, imipenem inhibited the Type I beta-lactamase produced by Enterobacter cloacae. Other beta-lactamases from gram-negative bacilli were also inhibited by high concentrations of imipenem.

Effect of refrigerated storage on cefaclor in Mueller-Hinton agar

Cefaclor is less stable than most cephalosporins in media at 35 degrees C. We demonstrated that the activity of cefaclor in Mueller-Hinton agar diminishes continuously at 4 degrees C, resulting in a loss of two-thirds of the activity within 21 days. We recommend that agar dilution plates for this cephalosporin be prepared on the day of their use.