Antimicrobial susceptibility of coagulase-negative staphylococci

Combinations of vancomycin and beta-lactams are synergistic against staphylococci with reduced susceptibilities to vancomycin

Evidence of synergism between combinations of vancomycin and beta-lactam antibiotics against 59 isolates of methicillin-resistant staphylococci (Staphylococcus aureus, Staphylococcus epidermidis, and Staphylococcus haemolyticus) for which vancomycin MICs ranged from 1 to 16 microg/ml were tested by broth microdilution checkerboard, disk diffusion, agar dilution, and time-kill antimicrobial susceptibility tests. The combination of vancomycin and oxacillin demonstrated synergy by all test methods against 30 of 59 isolates; no antagonism was seen. Synergy with vancomycin was also found by modified disk diffusion testing for ceftriaxone, ceftazidime, cefpodoxime, and amoxicillin-clavulanate but not for aztreonam. Evidence of synergy correlated directly with vancomycin MICs. The efficacy of vancomycin given alone and in combination with nafcillin was tested in the rabbit model of experimental endocarditis caused by three clinical isolates of glycopeptide-intermediate-susceptible S. aureus (GISA) (isolates HIP5827, HIP5836, and MU50). Two of the GISA isolates (isolates MU50 and HIP5836) were extremely virulent in this model, with 27 of 42 (64%) animals dying during the 3-day trial. Therapy with either vancomycin or nafcillin given as a single agent was ineffective for animals infected with HIP5827 or MU50. However, the combination of vancomycin and nafcillin resulted in a mean reduction of 4.52 log10 CFU/g of aortic valvular vegetations per g compared to the reduction for controls for animals infected with HIP5827 and a reduction of 4. 15 log10 CFU/g for animals infected with MU50. Renal abscesses caused by HIP5827 were sterilized significantly better with the combination of vancomycin and nafcillin than by either treatment alone. We conclude that the combination of vancomycin and beta-lactams with antistaphylococcal activity is an effective regimen for the treatment of infections with clinical strains of staphylococci which demonstrate reduced susceptibility to glycopeptides.

Topoisomerase sequences of coagulase-negative staphylococcal isolates resistant to ciprofloxacin or trovafloxacin

Coagulase-negative staphylococcal isolates (n = 188) were screened for susceptibility to oxacillin, ciprofloxacin, and trovafloxacin, a new fluoroquinolone. At an oxacillin concentration of >/=4 microg/ml, 43% were methicillin resistant; of these, 70% were ciprofloxacin resistant (MIC, >/=4 microg/ml). Of the methicillin-resistant, ciprofloxacin-resistant isolates, 46% were susceptible to </=2 microg of trovafloxacin per ml and 32% were susceptible to </=1 microg of trovafloxacin per ml. Sixteen isolates, including twelve that expressed fluoroquinolone resistance, were chosen for detailed analysis. Identification of species by rRNA sequencing revealed a preponderance of Staphylococcus haemolyticus and S. hominis among fluoroquinolone-resistant strains. Segments of genes (gyrA and grlA) encoding DNA gyrase and DNA topoisomerase IV were sequenced. Considerable interspecies variation was noted, mainly involving noncoding nucleotide changes. Intraspecies variation consisted of coding changes associated with fluoroquinolone resistance. As for S. aureus, ciprofloxacin resistance (MIC, >/=8 microg/ml) and increased trovafloxacin MICs (0.25 to 2 microg/ml) could be conferred by the combined presence of single mutations in each gyrA and grlA gene. Trovafloxacin MICs of >/=8 microg/ml also occurred, but these required an additional mutation in grlA.

Identification and antibiotic susceptibility of coagulase negative staphylococci isolated in corneal/external infections

AIMS

To identify and determine antibiotic susceptibility of coagulase negative staphylococci (CoNS) isolated from patients with chronic blepharitis, purulent conjunctivitis, and suppurative keratitis.

METHODS

A retrospective review of all culture positive cases of chronic blepharitis, purulent conjunctivitis, and suppurative keratitis between July 1995 and December 1996 was performed. Cases in which CoNS were the sole isolates were analysed. Species identification was performed by using a commercially available standardised biochemical test system. Antibiotic susceptibility to penicillin, gentamicin, tetracycline, erythromycin, ciprofloxacin, and teicoplanin was determined by agar disc diffusion (Kirby-Bauer method). Teicoplanin resistance was confirmed by agar dilution.

RESULTS

42 Staphylococcus epidermidis, four S warneri, three S capitis, two S hominis, one each of S xylosus, S simulans, S equorum, and S lugdunensis were identified. 37 CoNS were penicillin resistant, 12 gentamicin resistant, 28 tetracycline resistant, 18 erythromycin resistant, four ciprofloxacin resistant, and one teicoplanin resistant (MIC, 32 microg/ml). In total, 16 strains were resistant to three or more antibiotics.

CONCLUSION

Species of CoNS apart from S epidermidis may be isolated from patients with corneal and external infection. Antibiotic susceptibility of CoNS is unpredictable and multiresistant strains are common. As a result, antibiotic susceptibility testing should be performed in all cases of clinically significant ocular infections caused by CoNS.

Evidence for nasal carriage of methicillin-resistant staphylococci colonizing intravascular devices

Nasal surveillance cultures were performed for 54 patients exhibiting >/=10(3) CFU of methicillin-resistant coagulase-negative staphylococci per ml in central venous catheter (CVC) rinse cultures over a 6-month period. Forty-two of the nasal cultures yielded growth of methicillin-resistant coagulase-negative staphylococci, and 33 of the 42 cultures contained organisms that belonged to the same species as the CVC isolates. Of the 33 same-species isolates, 20 appeared to be identical strains by pulsed-field gel electrophoresis analysis. These data suggest that measures should be taken to reduce cross-contamination between the respiratory tract and intravascular devices. However, the potential interest in detecting methicillin-resistant coagulase-negative staphylococcus carriage in high-risk patients is hampered by the lack of sensitivity of nasal surveillance cultures.

Bovine mastitis in Finland in 1988 and 1995--changes in prevalence and antimicrobial resistance

Two surveys were carried out (during 1988 and 1995) to estimate the prevalence of bovine mastitis in Finland. In 1988, 17,111 quarter milk samples were obtained from 4495 cows, and in 1995 the corresponding figures were 10,410 and 2648. Antimicrobial susceptibility of mastitis pathogens was studied. Prevalence of mastitis on cow basis decreased from 47.8% in 1988 to 37.8% in 1995. Staphylococci was the largest group of pathogens isolated. The proportion of Staphylococcus aureus decreased and that of coagulase-negative staphylococci (CNS) increased. The proportion of strains resistant to at least one antibacterial drug increased with regard to S. aureus from 36.9% in 1988, to 63.6% in 1995 and with CNS from 26.6% to 49.7%. Most of the increase in antibacterial resistance was due to a higher number of beta-lactamase producing strains. Multiresistance also increased, but it was proportional to the overall increase in resistance. All the predominant mastitis streptococci were susceptible to beta-lactams tested.

Urosepsis in the critical care unit

Critical care unit patients show a higher risk of developing a bloodstream infection than ward patients. The urinary tract is the main source of hospital-acquired secondary bloodstream infection. Nosocomial urinary tract infection is promoted by bladder catheterization in the vast majority of cases. Aerobic gram-negative bacilli are the prevalent agents of bloodstream infection secondary to a nosocomial urinary tract infection. Sepsis and septic shock are severe complications of these infections in the critical care patient. Management of patients with a septic process of urinary source calls for the combination of adequate life-supporting care, an appropriate antibiotic therapy, and innovative adjunctive measures. Accurate catheter care is the best measure to adopt for the prevention of urosepsis.

Staphylococcus aureus and coagulase-negative staphylococci from blood stream infections: frequency of occurrence, antimicrobial susceptibility, and molecular (mecA) characterization of oxacillin resistance in the SCOPE program

Staphylococci are major causes of nosocomial blood stream infection. The recently completed SCOPE Surveillance Program found that coagulase-negative staphylococci (CoNS) and Staphylococcus aureus were the first and second most common etiologic agents, respectively, causing nosocomial blood stream infection in the USA. The frequency of oxacillin resistance was 68% among 1553 strains of CoNS and 26% among 787 strains of S. aureus in this study. Extended susceptibility profiles were generated for a subset of 150 S. aureus and 300 CoNS against 16 antimicrobial agents. Oxacillin-susceptible strains of both CoNS and S. aureus were uniformly susceptible to beta-lactam agents with the exception of ampicillin and penicillin. Oxacillin-susceptible S. aureus were also highly susceptible to the fluoroquinolones, aminoglycosides, and trimethoprim/sulfamethoxazole. The oxacillin-susceptible CoNS were less susceptible to these agents, and only glycopeptides were reliably active against oxacillin-resistant strains. PCR detection of the mecA gene was used to scrutinize current NCCLS interpretive breakpoint MICs for determining susceptibility or resistance to oxacillin. We found complete concordance between the presence or absence of mecA and the NCCLS oxacillin interpretive breakpoint categories for S. aureus. In contrast, the NCCLS breakpoints for oxacillin significantly underestimate the degree of true oxacillin resistance among CoNS. Using the presence of mecA as the reference standard, we detected 15.7% false susceptibility to oxacillin using a MIC susceptible breakpoint concentration of < or = 2 micrograms/mL. Lowering the oxacillin MIC breakpoint to < or = 0.25 microgram/mL for CoNS would greatly improve the accuracy of the MIC test performance. We found that both the current oxacillin disk test and the 30-microgram ceftizoxime disk test functioned quite well in predicting those strains of CoNS that contain mecA. These studies have demonstrated both a high level of antimicrobial resistance among nosocomial blood stream isolates of staphylococci as well as significant problems with the current NCCLS breakpoints for oxacillin when testing CoNS.

Bovine mastitis in Finland in 1988 and 1995--changes in prevalence and antimicrobial resistance

Two surveys were carried out (during 1988 and 1995) to estimate the prevalence of bovine mastitis in Finland. In 1988, 17,111 quarter milk samples were obtained from 4495 cows, and in 1995 the corresponding figures were 10,410 and 2648. Antimicrobial susceptibility of mastitis pathogens was studied. Prevalence of mastitis on cow basis decreased from 47.8% in 1988 to 37.8% in 1995. Staphylococci was the largest group of pathogens isolated. The proportion of Staphylococcus aureus decreased and that of coagulase-negative staphylococci (CNS) increased. The proportion of strains resistant to at least one antibacterial drug increased with regard to S. aureus from 36.9% in 1988, to 63.6% in 1995 and with CNS from 26.6% to 49.7%. Most of the increase in antibacterial resistance was due to a higher number of beta-lactamase producing strains. Multiresistance also increased, but it was proportional to the overall increase in resistance. All the predominant mastitis streptococci were susceptible to beta-lactams tested.

Monitoring infective complications following hip fracture

Hip fracture affects more than 55,000 people in the UK each year and this number is increasing. Because of their advanced age and other risk factors, hip fracture patients are at risk of developing infection and a variety of other non-infective complications. Surveillance of superficial wound and deep joint infection is important because of the large number of patients involved and represents a good example of targeted surveillance. Furthermore this may be conducted as part of a quality control programme monitoring other interventions such as prophylaxis for vascular thrombosis. However, to carry this out successfully, a simple but efficient system for recording, collecting and analysing data is required and adequate post-discharge surveillance must be carried out.

Evaluation of Alamar colorimetric broth microdilution susceptibility testing method for staphylococci and enterococci

We compared the results of the Alamar broth microdilution susceptibility testing method with the results of the National Committee for Clinical Laboratory Standards reference broth microdilution method for 119 gram-positive organisms. The strains were tested for their susceptibilities to 20 antimicrobial agents. Only appropriate antimicrobial agents were evaluated for each species of bacteria. Absolute categorical agreement between the reference method and the test method was 91.5% for enterococci, 99.8% for oxacillin-susceptible staphylococci, and 97.4% for oxacillin-resistant staphylococci. Essential agreement (percent complete agreement plus percent minor errors) was > 99% for all organisms tested. The results for enterococci showed no very major errors, one major error with ofloxacin, and numerous minor errors with the quinolones. However, all except one of the minor errors were within +/- 1 log2 dilution of the reference result. For staphylococci, only 2 very major errors (one each with chloramphenicol and oxacillin), 1 major error (chloramphenicol), and 15 minor errors (multiple drugs) were observed. The Alamar colorimetric system was easy to use and the results were easy to read. It appears to be an acceptable method for antimicrobial susceptibility testing of staphylococci and enterococci.

The utility of non-beta-lactam antimicrobial MICs as markers to distinguish oxacillin-resistant from oxacillin-susceptible strains of Staphylococcus epidermidis

Among 6,068 strains of Staphylococcus epidermidis, 75.5% were oxacillin-resistant. Oxacillin-susceptible strains were more frequently susceptible to erythromycin, clindamycin, ciprofloxacin, trimethoprim/sulfamethoxazole, gentamicin, and tetracycline than oxacillin-resistant strains. With the exception of erythromycin, non-beta-lactam MICs were less discriminatory for identifying oxacillin-resistant strains with oxacillin MICs < or = 2 micrograms/ml than for those with oxacillin MICs > or = 4 micrograms/ml.

The emergent needs for basic research, education, and surveillance of antimicrobial resistance. Problems facing the report from the American Society for Microbiology Task Force on Antibiotic Resistance

The American Society for Microbiology (ASM) convened a task force to study the current prevelance of antibiotic resistance and the problems associated with it. The task force produced a series of recommendations centered around three key elements: (a) education of the physician and the public; (b) encouragement of more basic research directed to the development of new antimicrobials and vaccines; and (c) the setting-up of a national surveillance system to both confirm and monitor the extent of the problem. Since the publication of this report in 1995, progress has been slow. No "consortium-style" funding has been set aside and any initiatives, such as the first steps in an education program by the ASM, are still in their planning phases. The spirit of cooperation and trust needed to deal with this problem appears to be lacking.

Emergence of teicoplanin-resistant coagulase-negative staphylococci

Over a period of 5 years we have recovered 32 clinical isolates of coagulase-negative staphylococci (CoNS) exhibiting either decreased levels of susceptibility or true resistance to teicoplanin (MICs, 16 to 128 micrograms/ml); these isolates make up 0.55% of the total CoNS isolated by us. Twenty-nine of the strains were also methicillin resistant, and all were susceptible to vancomycin. Fourteen of the strains were Staphylococcus epidermidis, fourteen were Staphylococcus haemolyticus, and four were Staphylococcus hominis. In one case, a strain of S. haemolyticus was isolated with a vancomycin-resistant, teicoplanin-resistant Enterococcus faecalis strain. All strains were nosocomially acquired and were isolated from 17 different wards. Teicoplanin resistance occurred as a sporadic phenomenon, and none of the isolates were epidemiologically related. The isolates were from 30 patients, 13 of whom presented with true infections (43%). Five (38%) of the 13 patients with true infections had been previously treated with vancomycin. None of the infected patients were previously treated with teicoplanin. The in vivo development of resistance to teicoplanin among CoNS strains limits the therapy of infections by these microorganisms. There is a need for surveillance of nosocomial isolates of CoNS to determine resistance to glycopeptides.

Comparison of disk diffusion, the E test, and detection of mecA for determination of methicillin resistance in coagulase-negative staphylococci

The aim of this study was to find a reliable, fast, and simple alternative to the methicillin disk method for determination of methicillin resistance in coagulase-negative staphylococci, since results of this method are often difficult to read due to growth within the zone of inhibition. The sensitivity of 319 strains of coagulase-negative staphylococci to a 5 microgram methicillin disk on Mueller-Hinton agar using an incubation period of 48 h was compared with that of 1 microgram and 5 micrograms oxacillin disks on Mueller-Hinton agar with or without 2% NaCl, using an incubation period of 24 h. The detection of mecA (MecAgen) by the polymerase chain reaction was used as a standard. Minimum inhibitory concentrations were determined by means of the E test. Of the 225 mecA-positive strains, 190, 215, and 193 were resistant to 5 micrograms methicillin, 1 microgram oxacillin and 5 micrograms oxacillin disks on Mueller-Hinton agar, respectively, and 216, 218, and 223 were resistant on Mueller-Hinton agar with 2% NaCl. Of the 94 mecA-negative strains, 89, 93, and 94 were susceptible to 5 micrograms methicillin, 1 microgram oxacillin, and 5 micrograms oxacillin disks on Mueller-Hinton agar, respectively, and 92, 93, and 94 were susceptible on Mueller-Hinton agar with 2% NaCl. Using breakpoints of 2 micrograms/ml for oxacillin resistance and 8 micrograms/ml for methicillin resistance, the E test yielded sensitivities of 99.6 and 99.1% and specificities of 97.9 and 98.9% after 48 h of incubation. The 5 microgram oxacillin disk was faster and easier to read than the methicillin disk and correlated better with detection of mecA than the methicillin disk of the 1 microgram oxacillin disk.

Phenotypic detection of mec A-positive staphylococcal blood stream isolates: high accuracy of simple disk diffusion tests

Detection of oxacillin-resistance in staphylococci by phenotypic methods remains problematic. Although standardized susceptibility test methods are adequate for Staphylococcus aureus, many are less satisfactory for the coagulase-negative staphylococci (CNS). We have studied 108 consecutive blood culture isolates of staphylococci. The mec A gene was detected by PCR in one S. aureus and 55 CNS isolates. Susceptibility testing was performed as follows: oxacillin (1-microgram), ceftizoxime (30-microgram), and cephalothin (30-microgram) by disk diffusion; oxacillin, ceftizoxime, cephalothin, methicillin, ampicillin, ampicillin/ sulbactam, penicillin, cefazolin, imipenem, and meropenem by the broth microdilution method. In addition, isolates were tested by the oxacillin agar screen plate method. The single oxacillin-resistant S. aureus strain was detected by all oxacillin susceptibility test methods and by the ceftizoxime disk and MIC methods. Two oxacillin-susceptible S. aureus were intermediate (minor error) by ceftizoxime broth microdilution (MIC, 16 micrograms/mL). The most sensitive, simple phenotypic methods for detection of oxacillin-resistant CNS (mec A positive) were as follows: oxacillin disk diffusion at 98%, oxacillin screen plate at 91%, oxacillin broth microdilution at 87%, ceftizoxime disk diffusion at 100%, ceftizoxime broth microdilution at 87%, and methicillin broth microdilution at 83%. These results indicate that oxacillin and ceftizoxime disk diffusion tests are the most accurate phenotypic methods in routine clinical use for detection of oxacillin-resistant CNS. Oxacillin broth microdilution MIC testing (2% NaCl supplement) would perform more satisfactorily (100% sensitivity) with an adjusted interpretive breakpoint at < or = 0.5 microgram/mL, in contrast to the lower accuracy of the "so-called" reference agar screen test.

Stomatococcus mucilaginosus septicemia in leukemic patients

OBJECTIVE: To report an unexpectedly high number of cases of septicemia with Stomatococcus mucilaginosus, and try to identify predisposing factors. METHODS: All blood cultures obtained during 1991--93 from patients treated at the hematologic ward were bacteriologically identified. The medical records of patients with S. mucilaginosus-positive blood cultures were retrospectively reviewed and evaluated. The antibiotic susceptibility pattern and restriction fragment length polymorphism (RFLP) of S. mucilaginosus were tested. RESULTS: S. mucilaginosus blood isolates from patients with hematologic malignancies were found to be as common as isolates of Staphylococcus aureus. Eleven patients with myelogenous leukemia and isolation of S. mucilaginosus from the blood are reported on. One patient had concomitant meningitis. All patients were neutropenic and most had oral mucositis and had been given ciprofloxacin prophylaxis. S. mucilaginosus isolates from these patients were resistant to ciprofloxacin in contrast to isolates from patients who had received other prophylactic regimens and seven isolates found in healthy individuals not recently treated with antibiotics. The resistant S. mucilaginosus were found to be of diverse genetic origin as determined by RFLP. CONCLUSIONS: The appearance of resistant strains during ciprofloxacin prophylaxis may be a predisposing factor for S. mucilaginosus septicemia. There was no evidence of a nosocomial spread of S. mucilaginosus strains.

Emerging resistance to antimicrobial agents in gram-positive bacteria. Enterococci, staphylococci and nonpneumococcal streptococci

Staphylococci (Staphylococcus aureus and coagulase-negative Staphylococcus species) and enterococci are the aetiological organisms in 47 to 52% of nosocomial blood stream infections and approximately 30% of all nosocomial infections in the US. In European intensive care units, almost half of all infections are attributed to staphylococci. The streptococci have also become increasingly important because of the modified virulence of Streptococcus pyogenes strains, and the emerging role of the viridans group streptococci as a cause of potentially fatal bacteraemia in the neutropenic host. Resistance to available antimicrobial agents is increasing and includes, in particular, resistance to the glycopeptides (vancomycin and teicoplanin) amongst enterococci, resistance to penicillinase-resistant penicillins (oxacillin and methicillin) and fluoroquinolones (ciprofloxacin and ofloxacin) amongst staphylococci, and resistance to penicillin and some other beta-lactams amongst viridans group streptococci. New compounds for effective therapy of infection with antimicrobial-resistant Gram-positive species are needed urgently. To this end, the streptogramin combinations [quinupristin/dalfopristin (RP 59500; Synercid)], everninomycin derivatives (SCH 27899), oxazolidinones (U-100572, U-100766) and several newer fluoroquinolones (clinafloxacin, DU 6859a, grepafloxacin, levofloxacin, sparfloxacin, trovafloxacin) are under rapid development and clinical investigation.

Future prospects and therapeutic potential of streptogramins

Dramatic changes in the epidemiology and susceptibility patterns of Gram-positive cocci during the last decade have mandated new approaches to the management of many bacterial infections. For example, there has been a sharp increase in the incidence of infections caused by Staphylococcus aureus, particularly those resistant to methicillin (MRSA), and methicillin-resistant coagulase-negative staphylococci, particularly those associated with foreign bodies and indwelling medical devices. Additionally, the worldwide spread of Streptococcus pneumoniae strains resistant to penicillin and macrolides, and the emergence of enterococci (particularly Enterococcus faecium) resistant to vancomycin, teicoplanin and other antibiotics, present further therapeutic problems. New antibacterial agents are urgently required to meet the challenges posed by these epidemiological trends. The semisynthetic streptogramins, a unique class of antibacterials currently under development, offer promise in the treatment of such multiresistant infections. Possible future applications include treatment of infections caused by the following organisms: MRSA, enterococci resistant to vancomycin, macrolides or lincosamides; and beta-lactam-resistant streptococci. They may also prove useful as therapy for children with staphylococcal infection and patients with multiresistant infections who are unable to tolerate vancomycin, including patients with skin and soft tissue infections caused by Gram-positive pathogens, patients with osteomyelitis, foreign body associated infections, endocarditis and sepsis due to Gram-positive bacteria. Clinical trials are required to evaluate the efficacy and tolerability of streptogramins in these settings.

Current perspectives on glycopeptide resistance

In the last 5 years, clinical isolates of gram-positive bacteria with intrinsic or acquired resistance to glycopeptide antibiotics have been encountered increasingly. In many of these isolates, resistance arises from an alteration of the antibiotic target site, with the terminal D-alanyl-D-alanine moiety of peptidoglycan precursors being replaced by groups that do not bind glycopeptides. Although the criteria for defining resistance have been revised frequently, the reliable detection of low-level glycopeptide resistance remains problematic and is influenced by the method chosen. Glycopeptide-resistant enterococci have emerged as a particular problem in hospitals, where in addition to sporadic cases, clusters of infections with evidence of interpatient spread have occurred. Studies using molecular typing methods have implicated colonization of patients, staff carriage, and environmental contamination in the dissemination of these bacteria. Choice of antimicrobial therapy for infections caused by glycopeptide-resistant bacteria may be complicated by resistance to other antibiotics. Severe therapeutic difficulties are being encountered among patients infected with enterococci, with some infections being untreatable with currently available antibiotics.

Antimicrobial activity of fusidic acid and disk diffusion susceptibility testing criteria for gram-positive cocci

The in vitro activity of fusidic acid was assessed and was compared with those of cloxacillin, cefamandole, vancomycin, teicoplanin, ofloxacin, ciprofloxacin, pefloxacin, and fleroxacin against 500 gram-positive cocci: 151 Staphylococcus aureus, 197 coagulase-negative staphylococci, and 152 Enterococcus faecalis strains. All clinical isolates were concomitantly tested by disk diffusion and agar dilution procedures as outlined by the National Committee for Clinical Laboratory Standards. The results with fusidic acid were further analyzed by regression line and error rate-bounded methods. With control American Type Culture Collection organisms, the values were within the limits of the National Committee for Clinical Laboratory Standards or published limits. The incidence of resistance to fusidic acid was 0.7% for S. aureus, 2.5% for coagulase-negative staphylococci, and 99.3% for E. faecalis. The correlation coefficient between the results of disk diffusion and agar dilution tests with fusidic acid was 0.90. Current interpretive criteria for susceptibility to fusidic acid (i.e., MIC of < 2 micrograms/ml and inhibitory zone of 20 mm) gave 1% false susceptibility (all strains being E. faecalis). This error rate is practically eliminated if a zone diameter of 21 mm is considered the breakpoint for susceptibility.