Significance of methicillin tolerance in experimental staphylococcal endocarditis

Antibiotic tolerance among clinical isolates: mechanisms, detection, prevalence, and significance

SUMMARYAntibiotic treatment failures in the absence of resistance are not uncommon. Recently, attention has grown around the phenomenon of antibiotic tolerance, an underappreciated contributor to recalcitrant infections first detected in the 1970s. Tolerance describes the ability of a bacterial population to survive transient exposure to an otherwise lethal concentration of antibiotic without exhibiting resistance. With advances in genomics, we are gaining a better understanding of the molecular mechanisms behind tolerance, and several studies have sought to examine the clinical prevalence of tolerance. Attempts have also been made to assess the clinical significance of tolerance through in vivo infection models and prospective/retrospective clinical studies. Here, we review the data available on the molecular mechanisms, detection, prevalence, and clinical significance of genotypic tolerance that span ~50 years. We discuss the need for standardized methodology and interpretation criteria for tolerance detection and the impact that methodological inconsistencies have on our ability to accurately assess the scale of the problem. In terms of the clinical significance of tolerance, studies suggest that tolerance contributes to worse outcomes for patients (e.g., higher mortality, prolonged hospitalization), but historical data from animal models are varied. Furthermore, we lack the necessary information to effectively treat tolerant infections. Overall, while the tolerance field is gaining much-needed traction, the underlying clinical significance of tolerance that underpins all tolerance research is still far from clear and requires attention.

When antibiotics fail: a clinical and microbiological perspective on antibiotic tolerance and persistence of Staphylococcus aureus

Staphylococcus aureus is a major human pathogen causing a vast array of infections with significant mortality. Its versatile physiology enables it to adapt to various environments. Specific physiological changes are thought to underlie the frequent failure of antimicrobial therapy despite susceptibility in standard microbiological assays. Bacteria capable of surviving high antibiotic concentrations despite having a genetically susceptible background are described as 'antibiotic tolerant'. In this review, we put current knowledge on environmental triggers and molecular mechanisms of increased antibiotic survival of S. aureus into its clinical context. We discuss animal and clinical evidence of its significance and outline strategies to overcome infections with antibiotic-tolerant S. aureus.

Bactericidal activity of oxacillin and glycopeptides against Staphylococcus aureus in patients with endocarditis: looking for a relationship between tolerance and outcome

Background

There is no clear relationship between in vitro bactericidal activity tests and clinical outcome. We studied bactericidal activity of oxacillin, vancomycin and teicoplanin against Staphylococcus aureus isolates in patients with endocarditis and then we sought to determine if there was a relationship between in vitro bactericidal activity and clinical outcome.

Methods

Minimal bacteriostatic and minimal bactericidal concentrations were determined for Staphylococcus aureus strains isolated from patients with endocarditis following standardized methods. Medical records were reviewed retrospectively to collect data on antimicrobial susceptibility at admission, antimicrobial therapy, need for surgery, embolic events and outcome.

Results and Discussion

Sixty-two Staphylococcus aureus strains were studied in 62 patients with endocarditis. Overall, 91.9% definite, 21% methicillin resistant and 72.6% cured. Surgery was performed in 32.3% and embolic events were documented in 64.5%. Tolerance to oxacillin and teicoplanin was more common than vancomycin tolerance among methicillin susceptible Staphylococcus aureus. Among methicillin resistant Staphylococcus aureus teicoplanin was shown to have a higher rate of tolerance than vancomycin. No statistically significant differences on clinical outcome between oxacillin tolerant and oxacillin non tolerant Staphylococcus aureus infections were observed. Tolerance to oxacillin did not adversely affect clinical outcomes of patients with methicillin susceptible Staphylococcus aureus endocarditis treated with a combination of antimicrobials including oxacillin. The cure rate was significantly lower among patients with methicillin resistant Staphylococcus aureus endocarditis.

Conclusions

In vitro bactericidal test results were not valid predictors of clinical outcome. Physicians need to use additional parameters when treating patients with staphylococcal endocarditis.

Endocarditis caused by Staphylococcus aureus: A reappraisal of the epidemiologic, clinical, and pathologic manifestations with analysis of factors determining outcome

Staphylococcus aureus is the leading cause of infectious endocarditis and its mortality has remained high despite better diagnostic and therapeutic procedures over time. We conducted a retrospective review of 133 cases of definite S. aureus endocarditis seen at a single tertiary care hospital over 22 years to assess changes in the epidemiology and incidence of the infection, manifestations, outcome, risk factors for mortality, and impact of cardiac surgery on prognosis.Patients were classified into 2 groups: 1) right-sided endocarditis (64 patients) and 2) left-sided endocarditis (69 patients). While the number of cases of left-sided endocarditis remained steady at 1-3 cases per 10,000 admissions, the incidence of right-sided endocarditis, after a peak in the early 1990s, declined to almost disappear in 2001. Among the cases of right-sided endocarditis, we found 2 subsets of patients with different clinical features and prognosis: the first subset comprised 53 intravenous drug abusers, and the second subset comprised 11 patients with catheter-associated S. aureus bacteremia and endocarditis. Fifty-one patients were human immunodeficiency virus (HIV)-positive drug abusers, most of whom (80.3%) had right-sided endocarditis. We did not find differences in mortality between HIV-positive and HIV-negative individuals; mortality seemed to depend more on the site of the heart involved than on HIV status.Among the cases of left-sided endocarditis, the mitral valve was more commonly involved than the aortic valve (61% vs. 30%). Overall, 74% of patients with left-sided endocarditis developed 1 or more cardiac or extracardiac complication. In comparison, only 23.4% of patients with right-sided endocarditis developed complications.Prosthetic valve endocarditis (PVE) was hospital-acquired more frequently than native valve endocarditis (NVE). Patients with PVE had a shorter duration of symptoms until diagnosis and presented with or developed cardiac murmurs less frequently than patients with NVE. Cardiac failure (49%), renal failure (43%) and central nervous system (CNS) events (35%) were frequently observed in patients with both PVE and NVE. Valve replacement was more frequently needed and more rapidly performed in patients with PVE than in their counterparts with NVE.The overall mortality of patients with right-sided endocarditis was 17%. While the mortality of right-sided endocarditis in injection drug users was 3.7%, the mortality of patients with right-sided endocarditis associated with infected intravenous catheters was 82% (odds ratio [OR], 0.01; 95% confidence interval [CI], 0.001-0.07). For left-sided endocarditis mortality was 38% and was not significantly different in patients with NVE or PVE (OR, 0.65; 95% CI, 0.23-1.87). CNS complications were associated with mortality in both NVE (OR, 6.55; 95% CI, 1.78-24.04) and PVE (OR, 32; 95% CI, 2.63-465.40). Development of 2 or 3 complications was associated with an increased risk of mortality (OR, 5.59; 95% CI, 1.08-28.80 and OR, 9.25; 95% CI, 1.36-62.72 for 2 vs. 1 complication and for 3 vs. 2 complications, respectively).Surgical treatment did not significantly influence mortality in cases of NVE, (OR, 3.19; 95% CI, 0.76-13.38) but significantly improved the prognosis of patients with PVE (OR, 69; 95% CI, 2.89-1647.18).S. aureus endocarditis is an aggressive, often fatal, infection. The results of the current study suggest that valve replacement will improve the outcome of infection, particularly in patients with PVE.

Tests for bactericidal effects of antimicrobial agents: technical performance and clinical relevance

Bactericidal testing has been used for several decades as a guide for antimicrobial therapy of serious infections. Such testing is most frequently performed when bactericidal antimicrobial agent therapy is considered necessary (such as when treating infectious endocarditis or infection in an immunocompromised host). It has also been used to ensure that the infecting organism is killed by (not tolerant to) usually bactericidal compounds. However, few data are available to support the role of such tests in direct patient care. Several important variables affect the reproducibility of the test results; however, proposed reference methods are now available for performing the MBC test. With minor modifications, these can provide a standardized approach for laboratories that need to perform them. Currently, little evidence is available to support the routine use of such testing for the care of individual patients. However, testing of new (investigational) antimicrobial agents can be beneficial in determining their potential to provide bactericidal antimicrobial activity during clinical use. New methods to assess bactericidal activity are being developed, but as yet none have been rigorously tested in patient care settings; further, for most of these methods, little information is available as to which technical parameters affect their results. In clinical laboratories, all bactericidal tests must be performed with rigorously standardized techniques and adequate controls, bearing in mind the limitations of the currently available test procedures.

The continuing importance of staphylococci as major hospital pathogens

Rates of hospital-acquired staphylococcal infection have risen substantially in the United States over the last decade. Moreover, it has been shown that bloodstream infections caused by these organisms account for significant morbidity and mortality. It is likely that the changes in medical practice and in types of patient account for these changes, and current antibiotic therapy has helped select more resistant organisms. The increasing use of implantable and temporary medical devices and the increased use of immunosuppressive therapies correlate strongly with the rising incidence of these organisms. Likewise, more patients are predisposed to these infections because of poor immune status and factors favouring colonization. Intrinsic microbiological factors such as glycocalyx production and the presence of bacterial surface proteins allow these organisms to adhere to protein coated foreign bodies and basement membranes thus enabling them to initiate infection and cause disease. Moreover, virulence factors such as encapsulation, slime production and elaboration of extracellular enzymes aid their resistance to host defences. As a group, staphylococci are a leading cause of hospital-acquired infection, and species identification is required for both treatment and control.

Comparison of in vivo and in vitro activities of antibiotics with different modes of action against a tolerant and a non-tolerant Staphylococcus aureus strain

The antibacterial efficacies of 4 antibiotics with different modes of action against a penicillin-tolerant and a non-tolerant strain of Staphylococcus aureus were investigated. For the in vitro studies the minimum inhibitory concentration (MIC) and the minimum bacterial concentration (MBC) were determined and short-term growth experiments at different antibiotic concentrations were performed. For the in vivo studies, antibacterial efficacy in an experimental infection in normal and granulocytopenic mice was evaluated. For erythromycin, rifampicin and ciprofloxacin, there was no difference in the MIC and MBC values for the 2 strains. Benzylpenicillin had an MBC value for the tolerant strains which was 256 times higher than the MIC; with the non-tolerant strain there was no difference. EC50 values, calculated from the in vitro short-term growth curves, gave similar results. Only benzylpenicillin exhibited a difference in activity against the tolerant strain, as reflected by the EC50 that was 290 times the EC50 for the non-tolerant strain. Studies in normal and granulocytopenic mice gave similar results: benzylpenicillin was 268 times less active against the tolerant strain than against the non-tolerant strain. Erythromycin, rifampicin and ciprofloxacin were 2-3 times less active against the tolerant strain than against the non-tolerant strain. The presence of granulocytes is important for the antibacterial effect of all antibiotics studied, since in the absence of granulocytes higher doses of the antibiotics are needed in order to obtain the same antibacterial effect as when granulocytes are present.

Resistance of Streptococcus lactis mutants to beta-lactam antibiotics

Minimal inhibitory concentration and minimal bactericidal concentrations of penicillin and other beta-lactam antibiotics were determined for Streptococcus lactis in milk and trypteine soy broth. The values were always higher in milk than in broth. Minimal inhibitory concentration of penicillin was higher in solid than in liquid media. Two mutants resistant to penicillin and other beta-lactam antibiotics were obtained, and their growth rate, proteolysis, and acidification patterns determined. Tolerance toward these antibiotics was found when the mutants were grown in milk but not when they were grown in broth.

Discrepancies between MBC and actual killing of viridans group streptococci by cell-wall-active antibiotics

We determined the MBC of amoxicillin and vancomycin, two antibiotics advocated for treatment and prophylaxis of bacterial endocarditis, for 24 strains of viridans group streptococci isolated from patients with endocarditis. We found that the MIC of amoxicillin for all strains was less than or equal to 0.25 micrograms/ml and the MBC was either low (less than 0.5 micrograms/ml) in 6 nontolerant strains or high (greater than 128 micrograms/ml) in 18 tolerant strains. The MIC of vancomycin for the 24 strains was less than or equal to 1 microgram/ml, and the MBC was either low (less than 1 microgram/ml) for 3 nontolerant strains or high (greater than 128 micrograms/ml) for 21 tolerant strains. In addition to the MBC, we determined the actual reduction of the viable bacterial counts in each tube dilution after 24 h of incubation. This determination was made by subtracting the number of colonies observed on the subculture plate from the number of bacteria contained in the initial inoculum. For both antibiotics we found that the maximal reduction in viable counts was achieved at or very close to the MIC and did not increase with increasing antibiotic concentrations (up to 128 micrograms/ml). As expected, the six strains for which the amoxicillin MBC was less than 0.5 micrograms/ml and the three strains for which the vancomycin MBC was less than 1 microgram/ml had a reduction of viable counts of more than 3 log10 (greater than 99.9% killing). In contrast, among the strains defined as tolerant to amoxicillin and vancomycin, there were wide variations in the actual reduction of bacterial counts, ranging from 3 log10 to less than 1 log10. Therefore our observations suggest that the reduction of viable streptococcal counts reflects more accurately the bactericidal effect of amoxicillin and vancomycin than does the MBC, which artificially divides the strains into sensitive or tolerant strains.

Efficacy of penicillin G, flucloxacillin, cefazolin, fusidic acid, vancomycin, rifampicin and fosfomycin in muscular infections in mice due to Staphylococcus aureus

Two strains each of sensitive, penicillinase-producing, methicillin-resistant and "tolerant" Staphylococcus aureus were used to infect mice intramuscularly. The mice were then treated with three doses each of fosfomycin, vancomycin, rifampicin, fusidic acid, penicillin G, flucloxacillin or cefazolin intravenously. Infections due to sensitive strains were effectively treated with all antibiotics investigated except fusidic acid. Fosfomycin, vancomycin, rifampicin and flucloxacillin showed the best activity against penicillinase-producing strains. Fosfomycin and vancomycin were equally effective against infections due to methicillin-resistant S. aureus. Infections caused by "tolerant" strains again responded best to fosfomycin, vancomycin and rifampicin.

Oscillating tolerance in synchronized cultures of Staphylococcus aureus

Cells of synchronized cultures of Staphylococcus aureus showed an oscillating MBC/MIC ratio when tested with penicillin G. Although the MICs did not differ significantly throughout the cell cycle, the MBC was at its maximum when actively dividing cells were inoculated.

Staphylococcal endocarditis. Laboratory and clinical basis for antibiotic therapy

Antibiotic therapy for staphylococcal endocarditis is based on in vitro susceptibility, antibiotic efficacy in experimental endocarditis, and clinical experience. Native valve endocarditis due to Staphylococcus aureus in non-addicts is treated with four to six weeks of a penicillinase-resistant penicillin, a cephalosporin, or vancomycin. An aminoglycoside can be added for the initial three to five days, but longer-term multiple-drug therapy (adding an aminoglycoside and rifampin) is reserved for unresponsive infection. Right-sided native valve endocarditis in addicts usually responds to less vigorous therapy than that for native valve endocarditis in non-addicts. Vancomycin is the drug of choice for endocarditis due to methicillin-resistant S. aureus. Intrinsic methicillin-resistance in Staphylococcus epidermidis is often cryptic, requiring special tests for detection. Methicillin-resistant S. epidermidis is the major cause of prosthetic valve endocarditis. Vancomycin, rifampin, and gentamicin therapy for two weeks, followed by vancomycin plus rifampin, is recommended for treating this infection. Despite potent antimicrobial therapy, surgery is important in the therapy of complicated endocarditis, particularly prosthetic valve endocarditis.

Responses of tolerant and nontolerant Staphylococcus aureus strains to methicillin treatment in an experimental infection in mice

Staphylococcus aureus strains can be divided into tolerant and nontolerant strains on the basis of their survival in vitro in the presence of high concentrations of methicillin (greater than or equal to 64 micrograms/ml). A strain is defined as tolerant if more than 2% of the inoculum survives under these conditions. The response of five susceptible and five tolerant S. aureus strains to treatment with methicillin was studied in an experimental thigh infection in mice. Animals were treated with one and two injections of methicillin (2.5 mg per mouse). At the end of treatment, the number of CFUs in the thigh muscles infected with the susceptible strains was found to be significantly lower than that in the thigh muscles infected with the tolerant

Lack of reproducibility of macrodilution MBCs for Staphylococcus aureus

MBCs of methicillin, oxacillin, penicillin G, cephalothin, vancomycin, and gentamicin were determined by a standard broth macrodilution technique for 101 clinical isolates of methicillin-susceptible Staphylococcus aureus. Increased killing (more than 99.9%) was observed after 48 versus 24 h of incubation for many strains, and cross tolerance to antimicrobial bactericidal activity (less than 99.9% killing) was frequently observed among antimicrobial agents. However, these in vitro measurements of bactericidal activity against S. aureus were not reproducible.

Tolerance percentage as a criterion for the detection of tolerant Staphylococcus aureus strains

In this study, the degree of tolerance was determined in several populations of Staphylococcus aureus isolates. The degree of tolerance of a staphylococcal strain can be established in a reproducible way by exposing the strain to increasing concentrations of a beta-lactam antibiotic and determining the number of surviving bacteria at each concentration. The number of surviving bacteria was expressed as a fraction of the initial inoculum. By this technique, it appears that for each strain the value of the surviving fraction stabilized above a certain concentration of the antibiotic. This value was called the tolerance percentage of the strain. In 64 S. aureus strains isolated from blood cultures in 1982, the tolerance percentages, after exposure to methicillin, varied from less than or equal to 0.1 to 6; 28% of the strains showed a tolerance percentage of less than or equal to 0.1, and 12.5% showed a tolerance percentage of greater than or equal to 2. Similar tolerance percentages were found with cloxacillin, nafcillin, cephalothin, and penicillin. Strains with a tolerance percentage of greater than or equal to 2 showed slow killing and lysis in the presence of a high methicillin concentration. A tolerance percentage of 2 appeared to be the breakpoint between susceptible and tolerant strains. Older collections of S. aureus strains, dating from the years 1951 to 1953 and 1957 to 1958, also included strains with a survival percentage of greater than or equal to 2, thus indicating that tolerance of S. aureus to beta-lactam antibiotics is not a new phenomenon.

pH-dependent oxacillin tolerance of Staphylococcus aureus

A proportion of clinical isolates of Staphylococcus aureus exhibit resistance to bactericidal activity of certain antibiotics, despite normal susceptibility to inhibition. This phenomenon is termed "tolerance." The methodology used to determine tolerance varies greatly. To clarify the relationship between laboratory methodology and tolerance, we determined the minimal bactericidal concentrations and minimal inhibitory concentrations for 20 clinical isolates by two methods. Inocula were prepared by either 3-h growth of the organism in Mueller-Hinton broth or overnight (22 to 24 h) cultures in Trypticase soy broth (BBL Microbiology Systems, Cockeysville, Md.). All inocula were plated for colony counts and tested for pH. An American Type Culture Collection (Rockville, Md.) reference strain was included in all tests for standardization. Tolerance was defined by the strictest criterion, i.e., a minimal bactericidal concentration/minimal inhibitory concentration ratio of greater than or equal to 100. With the first method, none of the 20 isolates displayed tolerance (mean inoculum pH, 7.15). When inocula were grown in Trypticase soy broth with overnight incubation, 35% (7 of 20) showed tolerance (mean inoculum pH, 6.22). There was a significant association between the decreased bactericidal capacity at high oxacillin concentrations and overnight incubation in Trypticase soy broth (P less than 0.01). We suggest that tolerance in staphylococci is in some way related to the pH value of the inoculating culture. Such pH-induced tolerance may have a clinical corollary in sequestered infections where the pH is acidic.

Therapeutic significance of penicillin tolerance in experimental streptococcal endocarditis

Tolerance to penicillin exists among the viridans group of streptococci, but its therapeutic significance is unknown. We studied the effect of penicillin alone and in combination with streptomycin, in vivo and in vitro, on three strains of dextran-producing Streptococcus sanguis serotype II which possess widely various degrees of penicillin tolerance. In rabbits with experimental endocarditis, treatment with procaine penicillin (250 mg/kg intramuscularly twice daily for 5 days) decreased the number of viable organisms in valvular vegetations from 8.82 log10 +/- 0.98 CFU/g in untreated controls to 5.31 +/- 1.19 for a highly tolerant strain, 4.22 +/- 1.05 for a less tolerant strain, and 1.79 +/- 1.72 for a nontolerant strain (P less than or equal to 0.01 for comparison between any of the four groups). None of 36 rabbits infected with tolerant strains were cured by 5 days of treatment with penicillin, but 10 of 23 animals infected with the nontolerant strain were cured (P = 0.00002). When streptomycin was given in combination with penicillin, rabbits infected with the nontolerant strain were cured within 3 days, and rabbits infected with the tolerant strain were cured within 5 days. These findings indicate that tolerance can exert a critical influence on the response of S. sanguis to penicillin therapy in vivo and that the combination of penicillin plus streptomycin exerts a synergistic effect against tolerant as well as nontolerant organisms.

Penicillin therapy of experimental endocarditis induced by tolerant Streptococcus sanguis and nontolerant Streptococcus mitis

The response of tolerant Streptococcus sanguis and nontolerant Streptococcus mitis infections to penicillin therapy was compared in the rabbit model of endocarditis. The minimal inhibitory and bactericidal concentrations of penicillin were 0.1 and 0.1 mug/ml, respectively, for S. mitis and 0.05 and 6.2 mug/ml, respectively, for S. sanguis. Time-kill studies done in vitro with penicillin concentrations of 2 and 20 mug/ml demonstrated minimal killing of the tolerant strain, with a 3 log difference in survival between the two strains after 24 and 48 h. Both strains produced endocarditis with comparable bacterial densities on the valvular vegetations. Rabbits were treated with procaine penicillin G in two dosage regimens, 80,000 or 5,000 U/kg given every 8 h. There was no difference between bacterial densities in valvular vegetations removed from rabbits infected with either strain after 2, 4, or 6 days of treatment with the high-dose regimen (serum penicillin concentration at 0.5 h, 9.4 mug/ml), despite the fact that serum bactericidal activity against the tolerant strain at 0.5 h was minimal. With the low-dose penicillin regimen (serum concentration at 0.5 h, 2.5 mug/ml), therapy was significantly less effective in the tolerant group only after 6 days of treatment. Similar results were obtained when penicillin was administered in low and high doses to prevent infection. In this animal model of infection, penicillin tolerance was associated with a diminished response to penicillin therapy only when the dose was severely restricted. In the high-dose regimen, there was no difference in the response to penicillin therapy between animals infected with either strain, despite the presence of only minimal serum bactericidal activity in the rabbits infected with the tolerant strain.

Screening of Neisseria gonorrhoeae for tolerant response to beta-lactam antibiotics

Minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) of penicillin, ampicillin, cefoxitin, and cefuroxime were determined for 103 beta-lactamase-negative Neisseria gonorrhoeae clinical isolates belonging to five different auxotypes. MBC determinations were base on killing 99.9% of the inoculum after 24 h of incubation. The MBC/MIC ratio was less than or equal to 8 for ampicillin, cefoxitin, and cefuroxime in all 103 strains. Two isolates which were very susceptible to penicillin (MIC, less than or equal to 0.015 micrograms/ml) had MBCs which were considerably greater than the MICs (MBC/MIC ratios, 32 and 64) for penicillin. A beta-lactamase-negative resistant subpopulation having the same auxotype as the total population was isolated from each of these two strains. Killing curve studies were in agreement with the existence of susceptible and resistant subpopulations, which may explain the high MBC/MIC ratios.

Evaluation of ceforanide as treatment for staphylococcal and streptococcal endocarditis

Ceforanide administered parenterally twice daily was used as the sole agent to treat 17 patients with right-sided endocarditis due to Staphylococcus aureus or nonenterococcal streptococci. Fifteen patients were cured of their original infection. Two patients were withdrawn from the study. One patient was transferred to another hospital 4 days after ceforanide therapy was initiated, and the other was changed to a different antibiotic regimen when his viridans streptococcus proved tolerant to ceforanide. The intramuscular form of ceforanide was well tolerated. It was stopped in two patients after week 3 of therapy because of adverse effects, possibly related to the study drug. These findings resolved with discontinuation of the ceforanide, and no additional antimicrobial therapy was necessary. Two patients who continued to abuse drugs intravenously during the study developed bacteremia with new organisms and required additional antimicrobial therapy. Ceforanide proved to be a useful agent in the treatment of right-sided endocarditis due to susceptible S. aureus and nonenterococcal streptococci.

In vitro evaluation of cefoperazone

The activity of cefoperazone, a new broad-spectrum cephalosporin, was tested in vitro against 670 clinical isolates of gram-negative bacilli and gram-positive cocci. With the exception of Enterobacter spp., it inhibited the majority of all organisms tested at a concentration of 6.25 microgram/ml. Of particular interest is its good activity against Pseudomonas aeruginosa isolates which are usually very resistant to cephalosporins. When compared with other antibiotics, it was more active than any available cephalosporin against the Enterobacteriaceae, and its activity was comparable to the investigational drugs tested. Except against Pseudomonas, cefoperazone was less active than moxalactam (LY127935). No significant decrease in activity was noted in medium and pH variation studies. A considerable decrease in activity resulted when the size of the inoculum was incrased from 10(5) to 10(7) cells/ml. The minimal bactericidal concentrations were within one or two dilution values of the minimal inhibitory concentrations against the majority of isolates tested, except Staphylococcus aureus.