Attempted reduction of Abbott MS-2 false-susceptible errors for cefotaxime-resistant Pseudomonas aeruginosa by medium modification.

Thirty-five modifications of Abbott MS-2 growth medium were evaluated in an attempt to reduce the high frequency of false-susceptible errors produced by the MS-2 system when testing resistant Pseudomonas aeruginosa against cefotaxime. A hypotonic modification with 0.1% KNO3 reduced the errors from 41 to 20% for 23 resistant isolates. All hypertonic modifications proved unsatisfactory. No modification reduced false-susceptible errors to acceptable clinical levels.

Addition of NaCl to MS-2 system clinical cuvettes for combined salt tolerance and susceptibility testing for group D streptococci.

Sodium chloride was added directly to individual wells of MS-2 system clinical cuvettes for evaluating the feasibility of simultaneous susceptibility and salt tolerance testing for group D streptococcal isolates. Salt tolerance was correctly detected for all 177 enterococci tested; there was no false-positive results for 29 group D nonenterococcal or 45 viridans streptococcal isolates.

Evaluation of a leukocyte dip-stick test used for screening urine cultures.

A 2-min leukocyte esterase dip-stick test for pyuria was used to screen 409 consecutive clinical specimens received for culture. The leukocyte esterase dip-stick test proved to be a sensitive and inexpensive means of screening urine specimens for the absence of leukocytes and, by inference, for the absence of significant bacteruria.

False-susceptible results from the MS-2 system used for testing resistant Pseudomonas aeruginosa against two third-generation cephalosporins, moxalactam and cefotaxime.

The MS-2 system (Abbott Laboratories) was used for cefotaxime and moxalactam susceptibility testing of 111 Pseudomonas aeruginosa isolates collected from our burn treatment center. The frequency of very major errors was 52% for moxalactam and 32% for cefotaxime for all isolates requiring a minimal inhibitory concentration of greater than or equal to 64 micrograms/ml. For isolates requiring a minimal inhibitory concentration of 64 micrograms/ml, the very major errors were 78% for moxalactam and 57% for cefotaxime. Testing of an additional 44 isolates collected from another hospital confirmed that these results were not unique to nosocomial strains from within the burn center. The ratio of errors did not change when MS-2 results were compared with disk diffusion results. Colony count testing done on cuvettes which produced false-susceptible results indicated that little numerical change occurred during the 5- to 6-h test cycle, whereas microscopy of cuvette contents revealed metabolically active filamentous bacilli. The high frequency of false-susceptible results indicated that the MS-2 should not be used for testing clinical P. aeruginosa isolates for resistance to the two third-generation cephalosporins currently available for the MS-2. A comparison of the disk diffusion results indicated that neither moxalactam nor cefotaxime could be reliably used for predictive, class-concept testing of P. aeruginosa.

Enhanced detection of bacteremia with a new BACTEC resin blood culture medium.

In this multicenter study, 621 sets of blood culture specimens were drawn from 280 patients who were suspected of being septic and who were receiving antimicrobial therapy. Equal volumes of each specimen were inoculated into BACTEC 6B and 16B media. The 16B medium contained adsorbent and cationic resins for neutralizing the effects of the drugs. Of the 621 sets drawn, there were 72 positive cultures in 16B and 52 positive cultures in 6B. In 23 cases the organism was detected only in the 16B medium, and in 3 cases the organism was detected in 6B only. The remaining 49 positives were detected in both culture bottles. In 13 of these 49 cultures, detection in 16B was made between 1 and 5 days earlier than in 6B, whereas 3 of 49 specimens were detected 1 day earlier in 6B; the remaining 33 cultures became positive at approximately the same time in both media. There were a total of 43 patients with positive cultures in this study. Of these patients, 28 had sepsis detected in both the 16B and 6B media. The 6B medium alone detected an additional three cases of sepsis, and the 16B resin medium alone identified 12 additional cases. Supplementary culturing of samples from patients receiving antimicrobial therapy significantly increased the number of positive cultures and positive patients, as well as significantly shortening the time to positivity in these cultures.