Time to detection of positive BacT/Alert blood cultures and lack of need for routine subculture of 5- to 7-day negative cultures.

Consecutive BacT/Alert blood cultures which were instrument negative following a 7-day incubation were subcultured. Eighteen (0.2%) of 11,476 bottles had growth on subculture. Eleven of these eighteen isolates were considered contaminants on the basis of the identity of the organism and lack of other positive blood cultures from the same patient. In addition, analysis of time to instrument detection for approximately 2,900 positive blood cultures indicates that 5 or 6 days of incubation is sufficient for the routine detection of clinically significant organisms from BacT/Alert blood cultures. These data indicate that subculture of 5- to 7-day instrument-negative BacT/Alert blood culture bottles is not necessary.

Reliability of latex agglutination tests for identification of Staphylococcus aureus resistant to oxacillin.

Commercial latex agglutination tests (LATs) for the simultaneous detection of clumping factor and protein A are gaining increased acceptance as a means of identifying Staphylococcus aureus. We evaluated two LATs (Accu-Staph; Carr-Scarborough, Decatur, Ga.; Staphaurex; Wellcome, Dartford, England) with particular emphasis on their ability to correctly identify oxacillin-resistant S. aureus. We tested 59 oxacillin-resistant S. aureus, 136 oxacillin-susceptible S. aureus, and 92 coagulase-negative staphylococcal strains with the two LATs and with thermonuclease, slide clumping factor, tube coagulase, and protein A hemagglutination tests. Clumping factor and protein A were present in 96.9 and 82.1% of our S. aureus strains, respectively. Accu-Staph correctly identified 92.8% and Staphaurex correctly identified 91.3% of S. aureus strains. No significant difference in LAT positivity rates, presence of clumping factor, or presence of protein A was found between oxacillin-resistant and -susceptible S. aureus. Overall, there were 31 false-negative LATs for 20 S. aureus strains, 14 with Accu-Staph and 17 with Staphaurex. Ninety-five percent of these strains possessed either clumping factor or protein A or both when these factors were determined independently. There were five false-positive LATs for four strains of coagulase-negative staphylococci (three Staphylococcus epidermidis and one Staphylococcus warneri), four with Accu-Staph and one with Staphaurex. Clumping factor was present in one S. warneri strain. Thus, the specificities of Accu-Staph, Staphaurex, and the clumping factor test were 95.6, 98.9, and 98.9%, respectively. Our results indicated that LATs identify oxacillin-resistant and -susceptible S. aureus equally well; however, they offer no greater sensitivity or specificity than the clumping factor test for identification of S. aureus.

Supplementary rapid biochemical test panel for the API 20E bacterial identification system.

The API 20E Analytical Profile Index typically suggests three or four conventional biochemical tests to complete the identification of strains either identified to genus only or that have multiple genera consistent with the profile number. We compiled a simple panel of eight rapid (4-h) tests that can substitute for the supplementary biochemical tests recommended by Analytab Products (Plainview, N.Y.). The rapid test panel (RTP) consisted of adonitol, cellobiose, lactose, raffinose, rhamnose, and xylose utilization, lysine decarboxylase activity, and motility. A total of 114 consecutive clinical isolates that required additional tests to complete the identifications were each tested with the complete RTP, as well as with the recommended conventional biochemicals. All discordant identifications were resolved by using an expanded series of conventional biochemical tests. Overall, 110 (96%) strains were identified to the correct genus, and 109 (95%) strains were identified to the correct species by using the RTP, as compared with 105 (92%) identified to the correct genus and 90 (79%) identified to the correct species with the recommended tests. The identifications based on the two supplementary test systems did not agree for 7 (6.1%) strains. Four discrepancies were resolved in favor of the RTP, and three were resolved in favor of the recommended tests. We were unable to identify five (4.4%) strains with the recommended tests and only one (0.9%) with the RTP. A majority (86%) of the test strains were identified to the species level with the RTP after only 4 h of incubation.

Clinical and bacteriologic evaluation of BACTEC resin-containing blood culture medium.

In a retrospective study of 1,143 blood culture sets, BACTEC aerobic (6B) and osmotically stabilized (8B) media were compared individually with resin-containing (16B) medium for the isolation of bacteria from the blood of patients receiving antimicrobial therapy. The 16B medium was found to detect significantly more positive cultures than either 6B (P less than 0.01) or 8B (P less than 0.001). For 22 of 25 isolates of Staphylococcus aureus from 18 patients, 16B medium provided the only means of recovery. All but one of these patients were receiving appropriate antimicrobial therapy, and 16 of 18 had a previous blood culture set positive for S. aureus that did not include a 16B bottle. There was no evidence of a change in antimicrobial therapy in response to a 16B positive culture in these patients. No significant increase in the recovery of other gram-positive or gram-negative bacteria or decrease in the time to radiometric detection of positive cultures as a result of using 16B medium was noted.

Incidence of thymidine-dependent enterococci detected on Mueller-Hinton agar with low thymidine content.

It was observed that 10% of urine culture isolates of enterococci tested for antimicrobial susceptibility failed to grow on commercially prepared Mueller-Hinton agar with low levels of thymidine and thymine. All strains could utilize exogenous thymidine and thymine and required only low levels (0.4-microgram disk) to support growth. All thymidine-thymine-requiring strains were resistant to trimethoprim-sulfamethoxazole.