Medium-dependent variation in bactericidal activity of antibiotics against susceptible Staphylococcus aureus

Copper intoxication in group B Streptococcus triggers transcriptional activation of the cop operon that contributes to enhanced virulence during acute infection

Bacteria can utilize Copper (Cu) as a trace element to support cellular processes; however, excess Cu can intoxicate bacteria. Here, we characterize the cop operon in group B streptococcus (GBS), and establish its role in evasion of Cu intoxication and the response to Cu stress on virulence. Growth of GBS mutants deficient in either the copA Cu exporter, or the copY repressor, were severely compromised in Cu-stress conditions. GBS survival of Cu stress reflected a mechanism of CopY de-repression of the CopA efflux system. However, neither mutant was attenuated for intracellular survival in macrophages. Analysis of global transcriptional responses to Cu by RNA-sequencing revealed a stress signature encompassing homeostasis of multiple metals. Genes induced by Cu stress included putative metal transporters for manganese import, whereas a system for iron export was repressed. In addition, copA promoted the ability of GBS to colonize the blood, liver and spleen of mice following disseminated infection. Together, these findings show that GBS copA mediates resistance to Cu intoxication, via regulation by the Cu-sensing transcriptional repressor, copY. Cu stress responses in GBS reflect a transcriptional signature that heightens virulence and represents an important part of the bacteria's ability to survive in different environments. Importance Understanding how bacteria manage cellular levels of metal ions, such as copper, helps to explain how microbial cells can survive in different stressful environments. We show how the opportunistic pathogen group B Streptococcus (GBS) achieves homeostasis of intracellular copper through the activities of the genes that comprise the cop operon, and describe how this helps GBS survive in stressful environments, including in the mammalian host during systemic disseminated infection.

Understanding tolerance to cell wall-active antibiotics

Antibiotic tolerance-the ability of bacteria to survive for an extended time in the presence of bactericidal antibiotics-is an understudied contributor to antibiotic treatment failure. Herein, I review the manifestations, mechanisms, and clinical relevance of tolerance to cell wall-active (CWA) antibiotics, one of the most important groups of antibiotics at the forefront of clinical use. I discuss definitions of tolerance and assays for tolerance detection, comprehensively discuss the mechanism of action of β-lactams and other CWA antibiotics, and then provide an overview of how cells mitigate the potentially lethal effects of CWA antibiotic-induced cell damage to become tolerant. Lastly, I discuss evidence for a role of CWA antibiotic tolerance in clinical antibiotic treatment failure.

Challenges and Hallmarks of Establishing Alkylacetylphosphonates as Probes of Bacterial 1-Deoxy-d-xylulose 5-Phosphate Synthase

1-Deoxy-d-xylulose 5-phosphate (DXP) synthase catalyzes the thiamin diphosphate (ThDP)-dependent formation of DXP from pyruvate and d-glyceraldehyde 3-phosphate. DXP is at a metabolic branch point in bacteria, feeding into the methylerythritol phosphate pathway to indispensable isoprenoids and acting as a precursor for biosynthesis of essential cofactors in central metabolism, pyridoxal phosphate and ThDP, the latter of which is also required for DXP synthase catalysis. DXP synthase follows a unique random sequential mechanism and possesses an unusually large active site. These features have guided the design of sterically demanding alkylacetylphosphonates (alkylAPs) toward the development of selective DXP synthase inhibitors. alkylAPs studied here display selective, low μM inhibitory activity against DXP synthase. They are weak inhibitors of bacterial growth in standard nutrient rich conditions. However, bacteria are significantly sensitized to most alkylAPs in defined minimal growth medium, with minimal inhibitory concentrations (MICs) ranging from low μM to low mM and influenced by alkyl-chain length. The longest analog (C₈) displays the weakest antimicrobial activity and is a substrate for efflux via AcrAB-TolC. The dependence of inhibitor potency on growth environment emphasizes the need for antimicrobial screening conditions that are relevant to the in vivo microbial microenvironment during infection. DXP synthase expression and thiamin supplementation studies offer support for DXP synthase as an intracellular target for some alkylAPs and reveal both the challenges and intriguing aspects of these approaches to study target engagement.

Clinical relevance of bacteriostatic versus bactericidal mechanisms of action in the treatment of Gram-positive bacterial infections

The distinction between bactericidal and bacteriostatic agents appears to be clear according to the in vitro definition, but this only applies under strict laboratory conditions and is inconsistent for a particular agent against all bacteria. The distinction is more arbitrary when agents are categorized in clinical situations. The supposed superiority of bactericidal agents over bacteriostatic agents is of little relevance when treating the vast majority of infections with gram-positive bacteria, particularly in patients with uncomplicated infections and noncompromised immune systems. Bacteriostatic agents (e.g., chloramphenicol, clindamycin, and linezolid) have been effectively used for treatment of endocarditis, meningitis, and osteomyelitis--indications that are often considered to require bactericidal activity. Although bacteriostatic/bactericidal data may provide valuable information on the potential action of antibacterial agents in vitro, it is necessary to combine this information with pharmacokinetic and pharmacodynamic data to provide more meaningful prediction of efficacy in vivo. The ultimate guide to treatment of any infection must be clinical outcome.

Time-kill evaluation of antimicrobial regimens against clinical isolates of penicillin-resistant Streptococcus pneumoniae

The rate of penicillin-resistant Streptococcus pneumoniae isolates in Spain is high. At present, penicillin and ceftriaxone are two drugs chosen for treating serious infections. In this study the bactericidal activity of four antimicrobial regimens against ten clinical isolates of S. pneumoniae (five with an intermediate resistance to penicillin and five highly resistant ones), was determined by means of kill kinetics studies using either penicillin, or ceftriaxone, in combination with vancomycin, or fosfomycin. The concentrations of the antimicrobial regimens (MICs 4x, 1x and 1/4x) were within possible physiological levels. While the combinations of penicillin, or ceftriaxone, plus vancomycin showed a significant increase in bactericidal activity, the bacterial reductions obtained in combination with fosfomycin were greater, achieving synergistic effects. These results suggest that in vivo trials with a regimen composed of ceftriaxone and fosfomycin would be worthwhile.

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.

Ampicillin killing curve patterns for ampicillin-susceptible nontypeable Haemophilus influenzae strains by the agar dilution plate count method

Ampicillin killing curve patterns for 20 strains of ampicillin-susceptible nontypeable Haemophilus influenzae were determined by the agar dilution plate count method. The paradoxical effect was detected in the 24-h killing curve patterns for each strain. For the biphasic effect, minimum survivor percentages (maximum killing) occurred over a narrow range of ampicillin concentrations immediately above the MIC, with survivor percentages then rising rapidly to peak at approximately 1-log10-unit increment higher. The 24-h minimum survivor percentages for the 20 strains ranged from approximately 0.01% (rapid killing) to greater than 10% (slow killing). In comparison with the previous results for typeable strains, the present findings suggest that nontypeable stains are, on average, killed much more slowly. Based on the initial 24-h killing curve patterns for the 20 strains, 4 strains were selected as putative representatives of the range of bactericidal responses encountered. These strains were then studied to examine the reproducibility of the 24-h patterns and to determine sequential killing curves. These patterns were found to be reproducible and served to characterize the relative killing responses of the strains. In the sequential studies of three of the four strains, tiny colonies having the gross and microscopic characteristics of L-forms were found to be present on the agar dilution plate count plates prior to the application of penicillinase at 48 and 72 h. Such colonies reverted to vegetative forms within 24 to 48 h after application of penicillinase to the panels. Of particular interest was the observation that the paradoxical effect was manifested both by the L-form colonies and by the reverted vegetative colonies. The late development of L-forms was observed for both rapidly and slowly killed strains.

Effect of human serum on the bactericidal activity of daptomycin and vancomycin against staphylococcal and enterococcal isolates as determined by time-kill kinetic studies

The bactericidal activity of daptomycin and vancomycin alone in cation-supplemented Mueller-Hinton broth and in human serum against clinical isolates of Staphylococcus aureus, Staphylococcus epidermidis, and Enterococcus faecalis was evaluated by exposing replicating microorganisms to concentrations ranging from 2 to 128 micrograms/ml for 24 hr. In addition, the possibility of emergence of resistance, the stability of each agent in the respective medium, and the percent of protein binding by human serum for each agent was evaluated. We found that a concentration of less than or equal to 8 micrograms/ml of daptomycin was sufficient to achieve bactericidal activity (greater than or equal to 99.9% killing of the inoculum) in cation-supplemented Mueller-Hinton broth for all staphylococcal isolates tested; a concentration of less than or equal to 16 micrograms/ml of daptomycin was required for bactericidal activity in cation-supplemented Mueller-Hinton broth for enterococcal isolates. In human serum, comparable bactericidal activity with daptomycin was achieved only with concentrations 8-16 times higher. A similar but less pronounced effect in human serum was seen for vancomycin. Neither daptomycin nor vancomycin was appreciably degraded in human serum over a 24-hr period. It is likely that the clinical efficacy of daptomycin in humans would be enhanced by higher dosing than has been studied to date.

Serum bactericidal test

The serum bactericidal test represents one of the few in vitro tests performed in the clinical microbiology laboratory that combines the interaction of the pathogen, the antimicrobial agent, and the patient. Although the use of such a test antedates the antimicrobial era, its performance, results, and interpretation have been subject to question and controversy. Much of the confusion concerning the serum bactericidal test can be avoided by an understanding of the various factors which influence bactericidal testing. In addition, the methodologic aspects of the serum bactericidal test have recently been addressed and should place this test on firmer ground. New information on the clinical utility of this test is becoming available; additional data are needed to establish more clearly the usefulness of the serum bactericidal test in specific infections. Such clinical trials from multiple centers will enable firmer recommendations for the future use of the serum bactericidal test.

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.

Comparative in vitro activity of four beta-lactam antibiotics in surgical wound fluid against staphylococci

Minimal inhibitory and bactericidal concentrations (MIC and MBC) of methicillin, dicloxacillin, cephalothin and cefuroxime were determined against 26 strains of staphylococci, using a broth dilution method with surgical wound fluid (WF) and Mueller-Hinton broth (MH) as test media. The staphylococci were all clinical isolates; 17 strains were coagulase-positive and 9 were coagulase-negative. The MIC-values of dicloxacillin and cephalothin estimated in MH were lower than those estimated in WF. For methicillin and cefuroxime the MIC-values in the two media were similar. With regard to MIC-values, the two penicillins were identical in WF, as were the two cephalosporins, but the cephalosporins were found to be more active than the penicillins, with an average of one dilution step. As regards the MBC to MIC ratios, no differences were found neither between the two media nor between the four antibiotics.

Investigations on the bactericidal activity of fosfomycin using the membrane filter-agar dilution method and the time-kill technique

Minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) for a 99% and a 99.9% kill can be reproducibly determined for fosfomycin by the agar dilution procedure when placing the inoculum on top of membrane filters. In 88.7% (85%) of 80 bacterial strains (Enterobacteriaceae, Pseudomonas aeruginosa, Staphylococcus aureus) examined we found a 99% (99.9%) kill at concentrations higher than the respective MIC by up to one dilution step. For 5% (10%), the bactericidal concentrations were more than two steps higher. Both the MIC and the MBC were influenced by the culture medium. The influence of the medium on fosfomycin activity was also shown by the time-kill curves. In nutrient broth, Mueller-Hinton broth, Iso-Sensitest broth and human plasma water (with and without glucose-6-phosphate) we found different killing rates, different killing maxima and different times of regrowth of the cultures. The bactericidal activity of fosfomycin against Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, and Pseudomonas aeruginosa ATCC 27853 was more pronounced in plasma water, nutrient broth, and Mueller-Hinton broth than in Iso-Sensitest broth.

Evaluation of oxacillin tolerance in Staphylococcus aureus by a novel method

A novel agar dilution plate-count procedure for the quantitative measurement of bacterial inhibition and killing is described. For Staphylococcus aureus versus oxacillin, by the agar dilution plate-count procedure it was found that only 1 of 20 clinical isolates and 1 of 7 allegedly tolerant reference isolates met the conventional definition of tolerance. By using inocula of 10(5) CFU per plate, most isolates were demonstrated to have subpopulations of cells which, although inhibited, persisted for 24 h in concentrations significantly above their MICs. The persister percentages at 24 h appeared to be strain dependent, and all persisters exhibited the paradoxical effect. For each isolate, the persister percentage markedly decreased after action by oxacillin for 48 h, and the paradoxical effect was greatly diminished. Our findings suggest that tolerance is an artificial and arbitrary concept that does not adequately characterize the inhibition and killing dynamics associated with the persister phenomenon.

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.

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.

Influence of growth medium on the in vitro activities of second- and third-generation cephalosporins against Streptococcus faecalis

The influence of culture medium of the MICs of eight cephalosporins for 45 strains of Streptococcus faecalis was investigated. The MICs of cephalothin, cefamandole, and cefoperazone were not substantially influenced by the type of culture medium used. In contrast, MICs of cefuroxime, ceftizoxime, cefotaxime, cefmenoxime, and ceftriaxone varied markedly with both the commercial brand and the blood content of the broth used. The use of Mueller-Hinton broths (from Oxoid Ltd., GIBCO Diagnostics, and Difco Laboratories) supplemented with 5% lysed sheep blood frequently resulted in MICs that were greater than or equal to 16 times lower than the MICs obtained with these same broths without blood. Similar, but less marked, patterns were observed when supplemented and unsupplemented brain heart infusion and Sceptor broths were used. The influence of the broth on MICs suggests a complex interaction between some cephalosporins, medium components, and organisms. The cephalosporins that were affected by media share an identical moiety at the 7-acyl position (cefuroxime is slightly different), but this structure is not shared by those cephalosporins that were not affected. This commonality in structure at the 7-acyl position may be partially responsible for the observed results.

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.

Factors influencing detection of tolerance in Staphylococcus aureus

The phenomenon of tolerance to cloxacillin and methicillin was studied in Staphylococcus aureus. It was demonstrated that the minimal bactericidal concentrations showed marked differences, depending on the method of detection used. These differences resulted from carry-over of the antibiotic to the subculture plates. When carry-over of the antibiotic was prevented by the addition of beta-lactamase to the nutrient medium, the antibiotics were no longer bactericidal. At a certain antibiotic concentration and at higher concentrations, however, each strain showed a certain survival percentage after 24 h. The tolerance percentage was determined for 15 strains. The values found for the individual strains ranged from less than 0.1 to 11% for cloxacillin and methicillin. since these percentages were reproducible within narrow limits, they could be regarded as a characteristic of the strains. The tolerance percentage was independent of the growth phase of the initial cultures.

Variables in demonstrating methicillin tolerance in Staphylococcus aureus strains

Certain technical considerations which affected the status of methicillin tolerance in Staphylococcus aureus strains were studied. Methods which consistently demonstrated tolerance or intolerance of a given strain were avoidance of inoculum splashing, use of stationary-phase inoculum, 24-h tube incubation, and minimization of antibiotic carry-over. These studies suggested a need for the establishment of a standardized reference for the determination of tolerance.

Rate of penicillin killing of Staphylococcus aureus and Autobac 1 susceptibility test results

A clinical isolate of Staphylococcus aureus interpreted as resistant to penicillin by the Autobac 1 susceptibility testing method (i.e., light-scattering index of 0.77) was found to be susceptible to penicillin by both the disk diffusion and broth dilution techniques. The growth rate of the clinical isolate during a 4-h incubation interval was similar to that of a known sensitive reference strain (S. aureus ATCC 25923) used as a control organism for the Autobac test. The bactericidal effect of penicillin was evaluated by measuring the rate of killing over a 4-h interval. The percentages of organisms surviving exposure to 5.0 or 2.5 U of penicillin G per ml (number of organisms recovered at 3 h/number of organisms introduced as inoculum) were 68 and 76%, respectively, for the clinical isolate and 15 and 21%, respectively, for the reference strain. After 24 h of incubation, penicillin was bactericidal for both strains. The need to increase the time of incubation for those S. aureus isolates resistant to penicillin after 3 h of standard incubation time in the Autobac system is discussed.

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.

Effect of storage and changes in bacterial growth phase and antibiotic concentrations on antimicrobial tolerance in Staphylococcus aureus

Forty clinical isolates of Staphylococcus aureus were tested for tolerance to oxacillin and cephalothin by broth dilution susceptibility tests and killing curves. Most experiments were carried out with stationary-phase inocula, but nine tolerant isolates were retested with log-phase inocula. All 40 isolates were retested in killing curves at double the antibiotic concentrations used in initial tests. Isolates were retested for tolerance to oxacillin after storage at --70 degrees C for 1 year. In broth dilution tests, 23 of 40 (57.5%) and 20 of 40 (50%) isolates were tolerant to oxacillin and cephalothin, respectively. By killing curves, 25 of 40 (62.5%) and 22 of 40 (55%) isolates were tolerant to oxacillin and cephalothin, respectively. When nine tolerant isolates were retested with log-phase inocula, none manifested tolerance. Only 25 to 30% of the isolates were tolerant in killing curves performed with oxacillin and cephalothin at concentrations double those used in initial tests. After storage at --70 degrees C for 1 year, only two-thirds of the isolates remained tolerant. In isolates that remained tolerant, the degree of tolerance diminished to about 25% of that observed in initial tests.

Method of reliable determination of minimal lethal antibiotic concentrations

The lack of a standardized, statistically reliable method for in vitro determinations of the minimal lethal or bactericidal concentrations of antibiotics has complicated analyses of isolates of Staphylococcus aureus which appear to be inhibited but not killed by the usual concentrations of cell wall-active antibiotics. We describe a method which identifies some of the covariants involved in determinations of minimal lethal concentrations. Lethality was defined as a 99.9% reduction in the initial inoculum of bacteria after 24 h of incubation. We limited the sample volume to 0.01 ml to minimize the inhibitory effect of antibiotic and corresponding rejection values, which detected lethality with a high degree of sensitivity and specificity. When the number of colonies on subculture was equal to or less than the rejection value, the antibiotic was considered lethal for the test organism. Rejection values encompassed initial inocula from 10(5) to 10(7) colony-forming units per ml for single and duplicate samples and allowed for 1 or 5% variability in pipette volumes and errors in initial inoculum determinations. This method was used to determine the minimal lethal concentrations of semi-synthetic penicillins for S. aureau isolates, one of which was tolerant to the killing action of penicillin.

Significance of methicillin tolerance in experimental staphylococcal endocarditis

Methicillin-tolerant staphylococci are organisms that are inhibited by low concentrations of the drug but are resistant to its bactericidal effects. The clinical significance of this in vitro phenomenon is unknown. An experimental model of staphylococcal endocarditis in rabbits was used to determine whether methicillin-tolerant staphylococci are particularly difficult to eradicate from infected heart valves. In vitro sensitivity testing was used to identify a tolerant and a nontolerant strain of Staphylococcus aureus. Rabbits prepared by the insertion of a polyethylene catheter into the left ventricle and injected with a tolerant strain survived significantly longer than those injected with a nontolerant strain. No significant differences were demonstrated in the prevention of endocarditis with a single prophylactic dose of methicillin, or in the treatment of established endocardial infection with multiple doses of methicillin. In this study, methicillin had the same activity against experimental endocarditis caused by a tolerant and a nontolerant strain of S. aureus.