James C. McLaughlin

Clinical assessment of anaerobic isolates from blood cultures

Patients at two tertiary-care medical centers were evaluated to determine the clinical significance of anaerobic isolates from their blood specimens and to identify whether aerobic and/or anaerobic conditions were necessary for the detection of Streptococcus pneumoniae isolates. Significant anaerobes were isolated from only 0.1% and 0.4% of all blood cultures collected. The majority of patients with significant anaerobes had clinical conditions in which anaerobes are known to cause infections. Of the S. pneumoniae organisms, 83% were isolated only from the aerobic bottles of a blood culture set. These data lend support to the recommendations for the selective ordering of anaerobic blood cultures without compromising the isolation of S. pneumoniae.

A comparison of the BioStar Strep A OIA™ rapid antigen assay, Group A Selective Strep Agar (ssA™), and Todd-Hewitt broth cultures for the detection of group A Streptococcus in an outpatient Family Practice setting

In some studies the BioStar Strep A OIA (optical immunoassay) has yielded inconsistent results, although originally it was reported to be more sensitive than conventional culture for the detection of group A Streptococcus (GAS). The Group A Selective Strep Agar with 5% sheep blood (ssA) incubated anaerobically has been reported to be more sensitive than conventional culture in the detection of GAS. We compared the BioStar Strep A OIA GAS rapid antigen detection kit to anaerobic culture on ssA with and without preincubation in Todd-Hewitt broth (THB) for the detection of GAS. From September 1995 through January 1996, throat swabs were collected in duplicate from 75 children (< or = 18 years) and 188 adults (> 18 years) who presented with pharyngitis in the outpatient University of New Mexico Family Practice Clinic. Thirty-one (12%) of the 263 cases were positive for GAS by culture and/or broth. Compared with anaerobic culture on the ssA, with and without preincubation in THB, the sensitivity, specificity, positive predictive value, and negative predictive value of the BioStar Strep A OIA were 77, 62, 22, and 95%, respectively. Compared with enrichment in THB followed by subculture on ssA and anaerobic incubation, the sensitivity, specificity, positive predictive value, and negative predictive value of direct culture on ssA and anaerobic incubation were 79, 99, 98, and 96%, respectively. All isolates were serologically grouped. The BioStar Strep A OIA is as sensitive as direct culture on ssA incubated anaerobically, but the low specificity and low positive predictive value when the OIA is used in low prevalence populations could lead to unnecessary antibiotic treatment.

Occurrence of Clostridium difficile toxin-associated gastroenteritis following antibiotic therapy for otitis media in young children

The pathogenesis of diarrhea following antibiotic therapy for otitis media in young children remains unknown. We performed a prospective study evaluating the incidence of diarrhea and Clostridium difficile toxin in 115 outpatients (ages 6 months to 6 years) with acute otitis media treated with ampicillin, amoxicillin or trimethoprim-sulfamethoxazole. In 21 patients younger than one year of age six of 11 developing diarrhea had toxin-positive stools compared with three of 10 without diarrhea (P = 0.39). In 94 patients between 13 months and 6 years of age three of 12 with diarrhea had toxin-positive stools compared with five of 82 without diarrhea (P = 0.06). Diarrhea was self-limited in all cases. Although the data suggest that C. difficile might have been associated with diarrhea in the older children, further studies will be required to confirm this finding.

Comparison of fixed concentration and fixed ratio options for dilution susceptibility testing of gram-negative bacilli to ampicillin and ampicillin/sulbactam

Ampicillin combined with sulbactam was tested at both fixed ratio (2:1 and 1:1) and fixed sulbactam concentrations (4 µg/ml, 8 µg/ml and 16 µg/ml) against 2440 consecutively isolated gram-negative bacilli. Sulbactam significantly enhanced the spectrum of ampicillin activity. Overall, at 8 µg/ml ampicillin inhibited 50 % of theEnterobacteriaceae isolates, whereas 69 % to 84 % of the isolates were inhibited by the various sulbactam combinations. The widest spectrum of activity for ampicillin/sulbactam was achieved by testing at a fixed sulbactam concentration of 16 µg/ml, followed by the 1:1 ratio and the fixed 8 µg/ml (84 %, 76 % and 74 % inhibited, respectively). The amount of sulbactam at the susceptible breakpoint concentrations of ampicillin markedly affected the percentage of susceptible strains. Combinations that include 8 µg/ml of sulbactam are suggested for consideration.

The role of chitin in the thermoprotection of Vibrio cholerae

Vibrio cholerae has been shown to be able to survive short periods of boiling. The potential role of chitin in the thermoprotection of Vibrio cholerae was determined. When V. cholerae 6706 (OI E1 Tor) cells were incubated at 37°C in the presence of chitin, the bacteria were able to grow in the absence of any other energy source. In phosphate-buffered saline, growth was not supported. The ability of six pathogenic strains to survive at 60°C was investigated. None of the isolates tested was able to survive at 60°C for 10 min regardless of whether chitin was present. Further investigation revealed that isolate 6706 was able to withstand 50°C for a period of 10 min; pre-incubation with chitin decreased the survival time at this temperature. The data presented here show that chitin does not provide thermoprotection.

Relative efficacy of tazobactam, sulbactam and clavulanic acid in enhancing the potency of ampicillin against clinical isolates of Enterobacteriaceae

Three beta-lactamase inhibitors were combined with ampicillin in a fixed 2:1 ratio. The activity of ampicillin was enhanced by tazobactam and by clavulanic acid, and to a lesser extent by sulbactam when tested against fresh clinical isolates ofEnterobacteriaceae. At a concentration of 8 µg/ml, ampicillin alone inhibited 49.6 % of 2,434 consecutive isolates of enteric bacilli compared to 81 % inhibited by ampicillin combined with tazobactam or clavulanic acid and 69.3 % inhibited by the sulbactam/ampicillin combination. A four-fold or greater reduction in ampicillin MICs was observed in comparable numbers of isolates with all three combinations, but the most marked effects were seen with strains that were highly resistant to ampicillin.

In vitro activity of ampicillin, amoxicillin, ampicillin-sulbactam, and amoxicillin-clavulanic acid against consecutive clinical isolates of enterobacteriaceae

Ampicillin was generally twice as active as amoxicillin against 2440 consecutive isolates of Enterobacteriaceae from five medical centers. When beta-lactamase inhibitors were added to the penicillins, there was a significant increase in susceptibility. The magnitude of the increased susceptibility to ampicillin-sulbactam (A-S) and amoxicillin-clavulanic (A-C) acid varied with the species and types of beta-lactamases elaborated. Although cross-susceptibility and cross-resistance between ampicillin and amoxicillin was nearly complete, major differences were documented between A-S and A-C with 6.7% of our consecutive isolates of Enterobacteriaceae. The clinical significance of these findings remains uncertain, but they may help explain some of the discrepancies occasionally observed by clinical microbiologists with the combination drugs.

Comparison of fixed concentration and fixed ratio options for testing susceptibility of gram-negative bacilli to piperacillin and piperacillin/tazobactam

Piperacillin combined with tazobactam was tested at both a fixed ratio (8:1) and fixed tazobactam concentration (4 µg/ml) against 2,685 consecutively isolated gram-negative bacilli and 56 highly piperacillin-resistant isolates. Tazobactam significantly enhanced the spectrum of piperacillin activity. Overall, at a concentration of 16 μg/ml piperacillin alone inhibited 78.8 % of theEnterobacteriaceae isolates compared to inhibition of 92.7 % and 95.5 % by the 8:1 ratio and fixed (4 µg/ml) tazobactam combinations, respectively. In MIC tests the two combination options performed comparably against both routine and highly piperacillin-resistant isolates. Synergistic inhibition was observed for comparable numbers of isolates with the two combination options, the most marked effect being seen in the more highly piperacillin-resistant isolates. Both testing options are supported by the available human pharmacokinetic data; however the 8:1 ratio of piperacillin to tazobactam may be preferable given that the clinical formulation contains the two compounds in an 8:1 ratio and this ratio is maintained in vivo.

Methods of measuring susceptibility of anaerobic bacteria to trovafloxacin, including quality control parameters.

Three methods approved by the National Committee for Clinical Laboratory Standards for testing the susceptibility of anaerobic bacteria were used to evaluate the fluoroquinolone, trovafloxacin. The methods gave essentially comparable results with 126 anaerobes and with three quality control strains. A collaborative study defined the quality control range for trovafloxacin MICs. Trovafloxacin had good in vitro activity against the more common anaerobes (MIC 90 <- 2.0 (Μg/ml).Trovafloxacin (CP-99,219) is a fluoroquinolone with a broad spectrum of antibacterial activity (1–3). Its in vitro spectrum includes many anaerobic bacteria (4).The National Committee for Clinical Laboratory Standards (NCCLS) currently recommends three different methods for testing the susceptibility of anaerobic bacteria (5). The standard reference method is an agar dilution procedure using Wilkins-Chalgren agar. Two alternative methods are an agar dilution technique using Brucella blood agar and a microdilution procedure using a broth version of Wilkins-Chalgren medium. It is important to determine whether these three procedures actually produce identical test results with each antimicrobial agent likely to be tested against anaerobes.

IN VITRO ACTIVITY OF CEFTRIAXONE AND OTHER CEPHALOSPORINS AGAINST 602 CLINICAL ISOLATES OF STAPHYLOCOCCI FROM GEOGRAPHICALLY DIVERSE MEDICAL CENTERS

The in vitro activity of ceftriaxone and six additional antimicrobial agents (ceftizoxime, cefoperazone, cefuroxime, fleroxacin, ciprofloxacin, and trimethoprim/sulfamethoxazole) was assessed or 602 recent clinical isolates of staphylococci from six geographically distinct medical centers in North America. All seven antimicrobial agents were active (90–100% of strains susceptible) against oxacillin-susceptible (OS) strains of Staphylococcus aureus (OSSA) and coagulase-negative staphylococci (OSCNS) but had limited activity against oxacillin resistant (OR) staphylococci. Our assessment of the in vitro antistaphylococcal activity of ceftriaxone against contemporary isolates of Staphylococcus aureus and coagulase-negative staphylococci indicates that the activity versus OS staphylococci has not changed over the past decade despite widespread use of the drug. It appears that these agents will continue to be useful for empiric therapy in those centers in which OR strains are uncommon.