The development of antimicrobial resistance in staphylococci

Staphylococci are among the most important aetiological agents of both community- and hospital-acquired infections. Staphylococcus aureus isolates resistant to penicillin because they produced beta-lactamase were isolated soon after the introduction of penicillin as a therapeutic agent. The production of beta-lactamase was mediated by a plasmid. The incidence of penicillin resistance in S. aureus increased during the succeeding decades until most clinical isolates were resistant to penicillin. During the 1940s and 1950s resistance to other antimicrobial agents was also demonstrated, usually in combination with penicillin resistance, thus creating multi-resistant strains. Methicillin, a penicillin that was essentially resistant to staphylococcal beta-lactamase, was introduced into clinical use in the early 1960s and this alleviated much of the problem of antimicrobial resistance in S. aureus, but methicillin-resistant strains were soon isolated. Although methicillin-resistant staphylococci had been a nosocomial problem in Europe in the 1960s and early-1970s, it was not until the mid-1970s that they became a problem in the United States. Since then the incidence of methicillin-resistant S. aureus has steadily increased. The methicillin-resistant S. aureus strains are usually multi-resistant including the beta-lactams and one or more of the following: aminoglycosides, macrolides, lincosamides, tetracyclines (usually not minocycline), and chloramphenicol. The methicillin-resistant S. aureus are generally susceptible to vancomycin, teicoplanin, rifampicin, coumermycin, minocycline, the quinolones, and sulphamethoxazole-trimethoprim. The coagulase-negative staphylococci are also important aetiological agents of nosocomial infections and the incidence of these infections has increased in recent years.(ABSTRACT TRUNCATED AT 250 WORDS)

National collaborative study of the prevalence of antimicrobial resistance among clinical isolates of Haemophilus influenzae

A total of 2,811 clinical isolates of Haemophilus influenzae were obtained during 1986 from 30 medical centers and one nationwide private independent laboratory in the United States. Among these, 757 (26.9%) were type b strains. The overall rate of beta-lactamase-mediated ampicillin resistance was 20.0%. Type b strains were approximately twice as likely as non-type b strains to produce beta-lactamase (31.7 versus 15.6%). The MICs of 12 antimicrobial agents were determined for all isolates. Ampicillin resistance among strains that lacked beta-lactamase activity was extremely uncommon (0.1%). Percentages of study isolates susceptible to cefamandole, cefaclor, cephalothin, and cephalexin were 98.7, 94.5, 87.3, and 43.3%, respectively. For 14 strains (0.5% of the total), chloramphenicol MICs were greater than or equal to 8.0 micrograms, and thus the strains were considered resistant. All of these resistant strains produced chloramphenicol acetyltransferase. In addition, all 14 strains were resistant to tetracycline; 11 produced beta-lactamase. The percentage of isolates susceptible to tetracycline was 97.7%. In contrast, erythromycin and sulfisoxazole were relatively inactive. The combination of erythromycin-sulfisoxazole (1/64) was more active than erythromycin alone but essentially equivalent in activity to sulfisoxazole alone. Finally, small numbers of clinical isolates of H. influenzae were resistant to trimethoprim-sulfamethoxazole and rifampin.

Susceptibility of multiply resistant Haemophilus influenzae to newer antimicrobial agents

One hundred and six isolates of Haemophilus influenzae from a national antimicrobial surveillance study demonstrated resistance to two or more of 10 primary antimicrobial agents by mechanisms other than or in addition to beta-lactamase. Of particular note were strains multiply resistant to ampicillin (by beta-lactamase production), chloramphenicol, trimethoprim/sulfamethoxazole, and tetracycline in various combinations. All of the aforementioned strains were shown to be highly susceptible to amoxicillin/clavulanate, the second generation cephalosporins cefuroxime and cefonicid, and the third generation cephalosporins cefotaxime, ceftizoxime, ceftriaxone, ceftazidime, moxalactam, and cefixime. However, 68 strains that demonstrated resistance or marginal susceptibility (MIC greater than or equal to 2 micrograms/ml) to ampicillin by mechanisms other than beta-lactamase, also demonstrated reduced susceptibility to amoxicillin/clavulanate (MICs up to 8 micrograms/ml) and the second generation cephalosporins (MICs up to 32 micrograms/ml). While the latter strains were susceptible to the third generation cephalosporins, MICs were often 10-fold higher than MICs of ampicillin susceptible isolates or of beta-lactamase producing isolates. All of the multiply antimicrobial-resistant strains were highly susceptible (MIC less than or equal to 0.25 micrograms/ml) to the two quinolones ciprofloxacin and pefloxacin.

Influence of cation supplements on activity of netilmicin against Pseudomonas aeruginosa in vitro and in vivo

In vitro studies were performed with 74 Pseudomonas aeruginosa isolates which were collected during a multicenter trial. The isolates were obtained from 70 patients who had been treated with netilmicin as the only antipseudomonal antibiotic. Clinically, 83% of the patients were cured or improved, and 64% of the Pseudomonas isolates were eliminated by chemotherapy. The 74 clinical isolates and 38 additional isolates with known mechanisms of aminoglycoside resistance were tested in three separate laboratories by disk diffusion methods and by microdilution tests with three broth media (Mueller-Hinton broth with full, half, and no cation supplements). Isolates that responded to netilmicin therapy and those that failed to respond were all susceptible by the disk test, and most were susceptible by microdilution tests with unsupplemented broth. However, over half of the clinical isolates appeared to be resistant when cations were added to the broth medium. Strains capable of producing enzymes that inactivate netilmicin were resistant by all methods tested. Broth dilution and agar dilution results were most comparable when half of the recommended cation supplements was added to Mueller-Hinton broth. Further consideration should be given to reducing the concentration of cations that are added to Mueller-Hinton broth when netilmicin susceptibility tests are being performed. However, additional studies with other aminoglycosides are needed before appropriate testing conditions can be standardized.

In vitro antimicrobial spectrum, occurrence of synergy, and recommendations for dilution susceptibility testing concentrations of the cefoperazone-sulbactam combination

Broth microdilution tests and an antimicrobial interaction (synergy) studies using various combinations of cefoperazone and sulbactam were performed in an effort to determine the most appropriate in vitro dilution test system. The test results with cefoperazone and sulbactam were categorized as synergistic (complete or partial) for nearly 80% of the strains isolated from clinical trial patients. The results indicate that the cefoperazone-sulbactam fixed ratio (2:1) maximized the cefoperazone spectrum of activity and best approximated the parenteral formulation of the drug. The cefoperazone-sulbactam combination had a greater antimicrobial activity than did the other comparison beta-lactams, except for imipenem, tested against strains of the family Enterobacteriaceae. To be consistent with the National Committee for Clinical Laboratory Standards interpretive breakpoints for cefoperazone alone, the following MIC breakpoints should be applied to the combination (2:1 ratio): less than or equal to 16/8 micrograms/ml, susceptible; 32/16 micrograms/ml, moderately susceptible; and greater than or equal to 64/32 micrograms/ml, resistant.

Quality control limits for teicoplanin susceptibility tests and confirmation of disk diffusion interpretive criteria

For monitoring the performance of teicoplanin susceptibility tests, the following quality control limits are recommended: Staphylococcus aureus ATCC 29213, MIC of 0.12 to 0.5 micrograms/ml; Enterococcus faecalis ATCC 29212, MIC of 0.06 to 0.25 micrograms/ml; and S. aureus ATCC 25923, zones 15 to 19 mm in diameter (30-micrograms disks). However, some lots of Mueller-Hinton agar provided unusually large zones of inhibition with both vancomycin and teicoplanin disks, and these lots should be excluded before routine use. Teicoplanin and vancomycin differed only in their activity against oxacillin-resistant strains of Staphylococcus haemolyticus, which had decreased susceptibility to teicoplanin but were fully susceptible to vancomycin. For tests with 30-micrograms teicoplanin disks, zones less than or equal to 10 and greater than or equal to 14 mm in diameter represent resistant and susceptible breakpoints, respectively.

Netilmicin disk susceptibility tests: effect of cations on the MIC correlates

When testing Pseudomonas aeruginosa against netilmicin, MICs were markedly affected by the concentration of cations added to the test medium. A susceptible disk test result (zone greater than or equal to 15 mm) corresponded to MIC less than or equal to 4.0 micrograms/ml in unsupplemented broth, less than or equal to 12 micrograms/ml in broth with half the usual amount of cations and less than or equal to 32 micrograms/ml in broth with the recommended concentration of cations. Tests with 30 micrograms netilmicin disks best predicted susceptibility as determined by MICs in broth without added cations. When the MICs were determined in cation supplemented broth, the number of interpretive discrepancies increased to an unacceptably high level.

Mechanism of aminoglycoside resistance among beta-lactam-resistant Escherichia coli in the United States

We examined aminoglycoside (AG) resistance in ampicillin-resistant Escherichia coli obtained from nine hospitals participating in the National Nosocomial Infections Study. The isolates were tested to 25 antimicrobials using broth microdilution methods. If the organism was intermediate or resistant to gentamicin, tobramycin, netilmicin, or amikacin, we determined the class of aminoglycoside-modifying enzyme (AME) using the phosphocellulose paper binding assay. Of 423 E. coli, 21 (5%) were intermediate or resistant to one or more of the AGs. All but two of these E. coli isolates had at least one AME. Twelve isolates had phosphotransferase (APH) enzymes; seven had adenyltransferase (ANT) enzymes (all ANT[2"]); and four had acetyltransferase (AAC) enzymes. The seven ANT[2"]-producing isolates were more likely to be acquired in the community than in the hospital (4/7 ANT[2"]-producing E. coli versus one of 14 of the other AG-resistant E. coli, p = 0.03, Fisher's exact test). These findings suggest that for E. coli resistant to both ampicillin and an AG, APH enzymes are the predominant AME class. Additionally, isolates with certain AMEs may be acquired both in the community and in the hospital.

Quality control limits for microdilution susceptibility tests with aztreonam, imipenem, ceftriaxone, ceftazidime, ceftizoxime, cefuroxime, and cefonicid

A six-laboratory collaborative study was performed in order to define quality control limits for microdilution tests with seven new beta-lactams (aztreonam, imipenem, ceftriaxone, ceftazidime, ceftizoxime, cefuroxime, and cefonicid). Four standard control strains were tested and the expected MIC limits for each of the appropriate drug-microorganism combinations were defined.

RO 23-6240 (AM-833), a new fluoroquinolone: in vitro antimicrobial activity and tentative disk diffusion interpretive criteria

The susceptibility of over 7000 recent clinical bacterial isolates to RO 23-6240, a new trifluorinated quinolone, was determined at four medical centers. Over 99% of Enterobacteriaceae and 97% of staphylococci were inhibited by less than or equal to 2.0 micrograms/ml of RO 23-6240. Only 71% of Pseudomonas spp. were inhibited by this concentration. Streptococci and enterococci were resistant to RO 23-6240. Clinical isolates of Haemophilus spp., pathogenic Neisseria spp., and Branhamella catarrhalis were inhibited by less than or equal to 0.25 micrograms/ml of RO 23-6240. This drug's antibacterial activity was comparable with that of enoxacin and norfloxacin, but was less than that of ciprofloxacin against most species. Using less than or equal to 2.0 micrograms/ml and greater than or equal to 8.0 micrograms/ml as the susceptible and resistant MIC breakpoints for RO 23-6240, the regression analysis-derived disk diffusion zone diameter breakpoints for the 5 micrograms disk are: Susceptible greater than or equal to 19 mm intermediate 16-18 mm, and resistant less than or equal to 15 mm.

CI-934, a new difluoroquinolone: in vitro antibacterial activity and proposed disk diffusion test interpretive criteria

The susceptibility of 7,763 clinical isolates at four medical centers to CI-934 and three comparative quinolones was tested. CI-934 was the most active compound against Gram-positive isolates, such as staphylococci (MIC 90 = 0.25 microgram/ml), and enterococci (MIC 90 = 0.5 microgram/ml). CI-934 was the least active of these drugs against Pseudomonas spp. (MIC 90 = greater than 8.0 micrograms/ml). Against all other organisms CI-934 was very effective, being most comparable with enoxacin. With a selected group of isolates, CI-934 demonstrated high activity against Haemophilus influenzae (MIC 90 = 0.06 microgram/ml), Neisseria meningitidis and N. gonorrhoeae (MIC 90 = 0.13 microgram/ml), Listeria monocytogenes (MIC 90 = 1.0 microgram/ml), methicillin-resistant staphylococci (MIC 90 = 0.13 microgram/ml), and modest activity against anaerobes. Disk diffusion susceptibility testing of CI-934 was evaluated using 3-, 5-, and 10-micrograms disks. Because of its lower interpretive error rate, the 3-micrograms disk is tentatively recommended. With less than or equal to 2 micrograms/ml and greater than 4 micrograms/ml as the susceptible and resistant MIC breakpoints, the corresponding 3-micrograms disk zone diameters breakpoints are greater than or equal to 15 mm and less than or equal to 11 mm. Because most isolates of Pseudomonas spp. are not susceptible to CI-934 and the zone size interpretive error rate is particularly high (33%) with Pseudomonas spp., we suggest that isolates of this genus not be tested for clinical purposes.

In vitro activity of a new macrolide, A-56268, compared with that of roxithromycin, erythromycin, and clindamycin

A new macrolide (A-56268) was found to be approximately twice as active as erythromycin and four to eight times more active than roxithromycin. All three macrolides were similar in their potency against anaerobes. Human plasma enhanced the antistaphylococcal activity of A-56268 and erythromycin but reduced the activities of roxithromycin and clindamycin.

Disk diffusion and disk elution tests with A-56268 and erythromycin

Zones of inhibition around 15 micrograms A-56268 disks were essentially the same size as those around 15 micrograms erythromycin disks. If the same MIC breakpoints are to be used for defining susceptible categories for both macrolides, interpretive zone size standards for erythromycin disk tests may also be used for A-56268 disk tests. Against anaerobic bacteria, the two macrolides were only marginally effective when broth dilution tests were incubated in anaerobic jars. The aerobically incubated thioglycolate broth disk elution test indicated that both macrolides were much more effective against anaerobes. Three 15 micrograms disks eluted in 5 ml thioglycolate provided satisfactory results.

Quality control limits for microdilution susceptibility tests with norfloxacin

A multilaboratory study was designed to define quality control limits for microdilution susceptibility tests with norfloxacin. The following limits were proposed: for Escherichia coli ATCC 25922, 0.03 to 0.125 micrograms/ml; for Pseudomonas aeruginosa ATCC 27853, 1.0 to 4.0 micrograms/ml; for Staphylococcus aureus ATCC 29213, 0.5 to 2.0 micrograms/ml; and for Streptococcus faecalis ATCC 29212, 2.0 to 8.0 micrograms/ml. The latter represents a change in the previously recommended control limits.

Antimicrobial activity of LY164846, a new oral cephalosporin, and recommendations for disk diffusion tests

LY164846 is a new oral cephalosporin with a limited spectrum of antimicrobial activity that includes staphylococci (other than methicillin-resistant), streptococci (other than enterococci), Haemophilus influenzae (beta-lactamase-negative and beta-lactamase-positive), Branhamella catarrhalis (beta-lactamase-negative and beta-lactamase-positive), and Neisseria species (beta-lactamase negative and beta-lactamase-positive). The tentative recommendations for susceptibility breakpoints are less than or equal to 4 micrograms/ml and greater than or equal to 19 mm for susceptible, 8 micrograms/ml and 15-18 mm for intermediate, and greater than or equal to 16 micrograms/ml and less than or equal to 14 mm for resistant.

Antimicrobial activity of cefmetazole (CS-1170) and recommendations for susceptibility testing by disk diffusion, dilution, and anaerobic methods

Cefmetazole, formerly CS-1170, was found to have antimicrobial activity slightly superior to that of cefoxitin but a clinically usable antimicrobial spectrum that should be considered identical to that of cefoxitin. Disk diffusion and dilution test methods with cefmetazole correlated highly (r, greater than or equal to 0.95) with cefoxitin results. The recommended 30-micrograms cefmetazole disk interpretive breakpoints for susceptibility and resistance were greater than or equal to 18 mm (MIC, less than or equal to 8.0 micrograms/ml) and less than or equal to 14 mm (MIC, greater than or equal to 32 micrograms/ml), respectively. Cefmetazole and cefoxitin should be considered to be in the same antimicrobial spectrum class, requiring separate testing for other cephalosporins such as cephalothin, cefamandole, cefuroxime, and cefotetan. Recommended interpretive criteria performed well for fastidious organisms (Haemophilus influenzae, Neisseria meningitidis, and Branhamella catarrhalis) and for broth microdilution tests with anaerobes. Cefmetazole and cefoxitin broth disk elution tests for anaerobic bacteria produced higher rates of false susceptibility results.

Microplate phosphocellulose binding assay for aminoglycoside-modifying enzymes

We modified the phosphocellulose binding assay for aminoglycoside-modifying enzymes (AMEs) by use of microdilution plates and a multichannel micropipette. Batteries of aminoglycoside substrates for screening organisms for the presence of AMEs as well as for subclassifying enzymes were prepared and stored in microdilution plates. When tested in parallel with the conventional tube reaction assay, the microplate assay yielded comparable radioactive counts and therefore equally correct identifications of AMEs in 32 isolates representing nine bacterial species. Other modifications, such as multichannel dispensing of crude enzyme preparations and radioisotopic precursors, provided a more rapid, convenient, and less expensive means of examining large collections of organisms for AMEs.

Screening pneumococci for penicillin resistance

Eighty-four pneumococci with various MICs of penicillin (38 with MICs of less than or equal to 0.06 micrograms/ml [susceptible], 35 with MICs of 0.12 to 1.0 micrograms/ml [relatively resistant], and 11 with MICs of greater than 1.0 micrograms/ml [resistant] ) were screened by a disk diffusion test using oxacillin and methicillin to see how well they distinguished penicillin-susceptible strains from those with decreased susceptibility to penicillin. The effects of Mueller-Hinton agar plus 5% sheep blood and Trypticase soy agar plus 5% sheep blood and two atmospheres, ambient air and a candle extinction jar (increased CO2), were compared. There were no obvious differences between the effects of the two media, but zones were generally larger in ambient air than in increased CO2. Although the oxacillin test can separate penicillin-susceptible and -resistant strains, it cannot separate penicillin-resistant from relatively penicillin-resistant strains by using the breakpoint of less than 20 mm recommended by the National Committee for Clinical Laboratory Standards. When the 20-mm breakpoint was applied to methicillin, 12% of the relatively resistant strains tested were erroneously classified as susceptible. When different breakpoints were used for methicillin, there was better separation of the two classes of penicillin-resistant isolates, but a few relatively resistant strains were still classified as susceptible. We recommend that oxacillin, not methicillin, be used as the screening agent with Mueller-Hinton sheep blood agar and ambient air incubation and that the breakpoint be less than 20 mm to indicate resistance or relative resistance.

The anti-microbial activity, beta-lactamase stability, and disk diffusion susceptibility testing of carumonam (RO 17-2301, AMA-1080), a new monobactam

Carumonam, a new monobactam, was found to have an anti-microbial spectrum similar to aztreonam. Its spectrum includes Enterobacteriaceae, Haemophilus influenzae, pathogenic Neisseria species, Pseudomonas aeruginosa, and some streptococci. Staphylococcus species, enterococci, and many other nonenteric gram-negative bacilli were not inhibited. Enterobacteriaceae resistant to cefoperazone (minimum inhibitory concentrations [MICs] greater than or equal to 32 mg/L) were more likely inhibited by carumonam (52% at less than or equal to 8.0 mg/L) than aztreonam (39%) or ceftazidime (35%). Dilution test methods on agar or in Mueller-Hinton broth produced similar results. Carumonam minimum bactericidal concentrations were usually the same or one dilution above the MIC. Carumonam and aztreonam were very stable to most chromosomal (P99, K1, K14) and plasmid-mediated beta-lactamases (TEM, OXA, PSE). The Klebsiella oxytoca enzymes hydrolyzed aztreonam at rates greater than or equal to fivefold higher than carumonam but at a rate less than 1% that of cephaloridine. The aztreonam MICs for these Klebsiella stains were greater than or equal to 32 mg/L, but the hydrolysis rates do not fully explain the high-grade resistance to aztreonam. In vitro susceptibility tests with 30-micrograms carumonam disks were found to be very predictive. Similar regression statistics were observed for aztreonam and cefotaxime. Recommendations for carumonam susceptibility testing are susceptible greater than or equal to 21 mm (less than or equal to 8.0 mg/L) and resistant less than or equal to 14 mm (greater than or equal to 32 mg/L). Cross-resistance analysis favors the independent testing of carumonam or aztreonam against gram-negative species other than Enterobacteriaceae and P. aeruginosa.

Tentative disk diffusion susceptibility interpretive criteria for pefloxacin

Standardized broth microdilution and disk diffusion susceptibility tests for pefloxacin were performed on 585 clinical isolates. The 5-micrograms pefloxacin disk is recommended, and the following breakpoints are proposed: susceptible, greater than or equal to 19 mm (MIC, less than or equal to 2.0 micrograms/ml); resistant, less than or equal to 15 mm (MIC, greater than 4.0 micrograms/ml); and intermediate, 16 to 18 mm.

Disk diffusion susceptibility testing of two macrolide antimicrobial agents: revised interpretive criteria for erythromycin and preliminary guidelines for roxithromycin (RU 965)

The 15-micrograms erythromycin disk was twice evaluated for interpretive accuracy against 417 and then 266 strains of gram-positive cocci, Neisseria meningitidis, and Haemophilus influenzae by using the criteria suggested by the National Committee for Clinical Laboratory Standards. These studies suggest a revision of streptococcal and Staphylococcus sp. interpretive guidelines to criteria (greater than or equal to 23 mm = susceptible, less than or equal to 13 mm = resistant) that are more compatible with in vivo erythromycin concentrations. It is also recommended that zone diameters be modified for H. influenzae (greater than or equal to 23 mm = susceptible, less than 22 mm = resistant) and that meningococci not be tested. A wide moderately susceptible category (1.0 to 4.0 micrograms/ml) would primarily include enterococcus strains and those organisms that would then respond only to parenteral administration of erythromycin. Roxithromycin (RU 965 or RU 28965), a new oxime ether erythromycin derivative, was also evaluated by investigator-prepared 15-micrograms disks and later with 30- and 60-micrograms commercial disks. Although roxithromycin was comparable to erythromycin in activity and regression line statistics, changes in the susceptible disk criteria were necessary because of superior roxithromycin serum concentrations and a longer serum half-life. Preliminary susceptible breakpoint criteria were greater than 21 mm = susceptible, 10 to 20 mm = indeterminate, and less than or equal to 9 mm = resistant. By using the recommended interpretive criteria for both macrolides, less than 98% absolute agreement was obtained, therefore suggesting the application of a spectrum class concept.

In vitro evaluation of cefixime (FK027, FR17027, CL284635): spectrum against recent clinical isolates, comparative antimicrobial activity, beta-lactamase stability, and preliminary susceptibility testing criteria

Cefixime, a new orally absorbed cephalosporin, was compared by in vitro testing with other oral beta-lactams, including cephalexin, cefaclor, cefuroxime, amoxicillin, and amoxicillin + clavulanate. Enterobacteriaceae were inhibited by lower concentrations of cefixime than any of the reference drugs; 90% and 95% were inhibited by less than or equal to 1.0 and less than or equal to 8.0 micrograms/ml, respectively. Cefixime was the least active among these drugs against staphylococci, with only 31% of 1106 strains inhibited by less than or equal to 8.0 micrograms/ml and less than 1% by less than or equal to 1.0 microgram/ml. Enterococci and pseudomonads were not susceptible to any of the drugs tested. Penicillin-resistant pneumococci were relatively resistant to cefixime, but penicillin-susceptible pneumococci were very susceptible to cefixime. Other streptococci were generally susceptible to all compounds tested, with relative activities of amoxicillin greater than cefaclor and cefuroxime greater than cefixime greater than cephalexin. Cefixime was inactive against Bacteroides species. A slight inoculum effect occurred with cefixime with inocolum concentrations varying from 10(5) to 10(6) colony forming units per milliliter, but this was more marked at 10(7) colony forming units per milliliter. Cefixime was resistant to hydrolysis by seven common beta-lactamases. It inhibited the hydrolysis of nitrocefin only by type 1 cephalosporinases. The disk diffusion zone diameter breakpoints for the 30-micrograms cefixime disk were determined by regression analysis to be greater than or equal to 27 mm (susceptible) and less than or equal to 23 mm (resistant), respectively corresponding to minimal inhibitory concentration breakpoints of less than or equal to 1.0 and greater than or equal to 4.0 micrograms/ml. Because of the high interpretive error rate (13.8%) and the occurrence of these breakpoints on the parabolic portion of the regression curve, we recommend further evaluation of cefixime disks with lower potencies.

The role of beta-lactamase in staphylococcal resistance to penicillinase-resistant penicillins and cephalosporins

We showed that most Staphylococcus aureus strains that have borderline or intermediate susceptibility to the penicillinase-resistant penicillins (PRPs) react this way because of the activity of their beta-lactamase on these antimicrobial agents. These strains produced large amounts of staphylococcal beta-lactamase that rapidly hydrolyzed penicillin and partially hydrolyzed the PRPs. Susceptibility to hydrolysis was penicillin greater than oxacillin greater than cephalothin greater than methicillin. The borderline results and the hydrolysis could be prevented by the beta-lactamase inhibitors clavulanic acid and sulbactam. For intrinsically methicillin-resistant (heteroresistant) S. aureus, the inhibitors reduced the penicillin MICs, but the strains remained resistant to all the beta-lactam antimicrobial agents, including penicillin. We conclude that the borderline in vitro susceptibility or resistance to PRPs in most of these S. aureus strains is mediated by beta-lactamase and they are not heteroresistant or intrinsically resistant. We do not know whether this in vitro resistance is expressed clinically.