RESUMEN

This study examines the potential of Bordetella bronchiseptica to act as a human pathogen. After encountering two patients from whom B. bronchiseptica was isolated, we searched the literature and found 23 reports in which a human infection was reported in association with B. bronchiseptica. As a basis for evaluating these cases, we summarize the literature about the current microbiological status of B. bronchiseptica, the pathology and pathogenic mechanisms associated with the microorganism, and the likelihood of it acting as a commensal or colonizer. From this review we conclude that B. bronchiseptica has been rarely isolated from humans despite their considerable exposure to animal sources. Evidence suggests that B. bronchiseptica may be rarely encountered as a commensal or colonizer of the respiratory tract of humans and rarely in association with infection. When found as a probable pathogen, most infections have been respiratory tract in origin and have occurred in severely compromised hosts.

Asunto(s)

RESUMEN

Ampicillin killing curve patterns for 20 strains of ampicillin-susceptible nontypeable Haemophilus influenzae were determined by the agar dilution plate count method. The paradoxical effect was detected in the 24-h killing curve patterns for each strain. For the biphasic effect, minimum survivor percentages (maximum killing) occurred over a narrow range of ampicillin concentrations immediately above the MIC, with survivor percentages then rising rapidly to peak at approximately 1-log10-unit increment higher. The 24-h minimum survivor percentages for the 20 strains ranged from approximately 0.01% (rapid killing) to greater than 10% (slow killing). In comparison with the previous results for typeable strains, the present findings suggest that nontypeable stains are, on average, killed much more slowly. Based on the initial 24-h killing curve patterns for the 20 strains, 4 strains were selected as putative representatives of the range of bactericidal responses encountered. These strains were then studied to examine the reproducibility of the 24-h patterns and to determine sequential killing curves. These patterns were found to be reproducible and served to characterize the relative killing responses of the strains. In the sequential studies of three of the four strains, tiny colonies having the gross and microscopic characteristics of L-forms were found to be present on the agar dilution plate count plates prior to the application of penicillinase at 48 and 72 h. Such colonies reverted to vegetative forms within 24 to 48 h after application of penicillinase to the panels. Of particular interest was the observation that the paradoxical effect was manifested both by the L-form colonies and by the reverted vegetative colonies. The late development of L-forms was observed for both rapidly and slowly killed strains.

Asunto(s)

RESUMEN

Quality control for in vitro antimicrobial susceptibility testing of Pseudomonas aeruginosa versus aminoglycosides is improved by P. aeruginosa ATCC 49189, which was developed in the Clinical Microbiology Laboratory at St. Paul-Ramsey Medical Center. This strain, used by us for daily testing for the past 6 years, requires MICs that approximate therapeutic concentrations, are midrange in most dilution schemes, and are stable and reproducible.

Asunto(s)

RESUMEN

In the course of using Mueller-Hinton agar with 1% Supplement C (SC) (Difco, Detroit, MI) as a susceptibility test medium for Haemophilus influenzae, one lot of SC was encountered whose use was associated with markedly increased ampicillin MICs. Acidimetric and chromogenic cephalosporin filter paper disc tests of SC failed to detect beta-lactamase activity. Macrobroth dilution MIC tests to determine substrate specificity showed SC to antagonize benzylpenicillin and ampicillin but not cephalothin, cefazolin, or cefaclor, with the antagonism being prevented by the addition of clavulanic acid. High pressure liquid chromatographic analysis of reference and reaction solutions of benzylpenicillin with SC showed almost complete degradation of benzylpenicillin to benzylpenicilloic acid after 24 hr at 37 degrees C. For two other lots of SC that had passed MIC quality control testing, similar high pressure liquid chromatographic studies demonstrated slow conversion of small amounts of benzylpenicillin to benzylpenicilloic acid. These findings indicate that the beta-lactam antagonism by SC was due to the presence of a contaminating beta-lactamase directed primarily toward the penicillins.

Asunto(s)

Asunto(s)

RESUMEN

In a prospective survey of 283 Neisseria gonorrhoeae isolated in St. Paul, Minnesota, 5% were found to produce beta-lactamase, and 11% were resistant to penicillin by chromosomal mediation, thus indicating a hyperendemic level of resistance according to Centers for Disease Control guidelines. A significant level of chromosomal resistance (30%) was noted for tetracycline, and MICs for spectinomycin approached the upper limits of susceptibility for a large number of isolates. The need for uniformity in agar dilution test methodology is discussed, and the variability in antimicrobial agent interpretive breakpoint criteria is illustrated by comparing penicillin breakpoints published by the Centers for Disease Control with those of the National Committee for Clinical Laboratory Standards as applied to the study isolates.

Asunto(s)

RESUMEN

The effect of increased temperature on Staphylococcus aureus during the inoculation step of the agar dilution plate count method was investigated as a possible cause of artificially high persister counts. For some isolates, exposure of the inoculum to increased temperature resulted in higher persister counts and diminution or loss of the paradoxical effect. The persister patterns for three representative S. aureus isolates are presented to illustrate the strain- and temperature-dependent nature of the phenomenon. For any isolate, the net effect appears to be caused by an interplay of temperature-induced inoculum loss and temperature-induced cell division cycle blockage. A modification of the agar dilution plate count inoculation step to circumvent such problems is described.

Asunto(s)

Asunto(s)

RESUMEN

An elderly woman with Streptococcus pneumoniae meningitis relatively resistant (minimum inhibitory concentration, [MIC] = 0.12 micrograms/mL) to penicillin is reported. The occurrence of penicillin-resistant pneumococcal infections is reviewed and management discussed. Because of the importance of recognition of resistant pneumococci, a state-wide clinical laboratory survey was conducted to determine the accuracy of susceptibility testing for this isolate. Of 111 laboratories completing the survey, only 26 performed the 1-microgram oxacillin disk test as recommended by the National Committee for Laboratory Standards (NCCLS). When laboratories were analyzed according to hospital size, the proficiency in performing the proper susceptibility testing was 55% (6 of 11) for hospitals with more than 400 beds versus 3% (2 of 58) for hospitals with fewer than 100 beds (P less than 0.0001 by Fisher's exact test). This contrasts with reported surveys by the College of American Pathologists (CAP), and reasons for this are explored. Guidelines for laboratory testing of S. pneumoniae are reviewed, and additional study of clinical proficiency with attention to laboratory size is recommended.

Asunto(s)

RESUMEN

The responses of 20 ampicillin-susceptible Haemophilus influenzae type b clinical isolates to the bactericidal action of ampicillin were studied by using a modified agar dilution plate count method. A well-defined paradoxical effect was observed in each of the 24-h killing curve patterns for 19 of the 20 isolates, the remaining isolate showing a less-well-defined but suggestive paradoxical effect after 48 h of ampicillin exposure. For each isolate, the lowest 24-h persister percentage representing maximum killing (paradoxical trough percentage) occurred over a narrow range of concentrations immediately above the MIC, with such paradoxical trough percentages for the 20 isolates ranging from greater than 0.1 to less than 0.001%. Three isolates selected to represent slow, intermediate, and rapid responses were investigated by repetition of 24-h studies and by determination of expanded killing curve patterns. Resultant agar dilution plate count killing curve patterns were found to be reproducible and strain dependent and served to characterize each isolate. The paradoxical effect became more distinct with the prolongation of ampicillin action. Maximum killing was again evident for a narrow range of ampicillin concentrations immediately above the MIC, with persister percentages rising rapidly over the next few ampicillin concentrations to peak at 1 to 2 log10 increments higher than trough percentages. Based on the broad range of responses observed for the 20 isolates, the consistent presence of the paradoxical effect, and the time-dependent nature of bactericidal action, we suggest that the MBC and MBC/MIC ratios are inadequate indices of bactericidal action and that the all-or-none concept of "antimicrobial tolerance" should be abandoned.

Asunto(s)

RESUMEN

Of 175 recent Minnesota Hemophilus influenzae type b isolates from systemic disease, 43 were found to be resistant to ampicillin (greater than or equal to 4 micrograms/mL [mg/L]), each of which produced beta-lactamase. Of the 132 ampicillin-susceptible isolates, 68 (52%), all beta-lactamase negative, had minimum inhibitory concentrations (MICs) of either 1 or 2 micrograms/mL (mg/L), indicating relative resistance if derived from cerebrospinal fluid (CSF) infections. From a review of the literature, and in agreement with the authors findings, ampicillin-resistant beta-lactamase-negative isolates are rare and are likely to be nontypeable, of respiratory origin, and with MICs in the low resistance range. For the 43 ampicillin-resistant isolates, percentages resistant to other agents were as follows: 0% chloramphenicol, 0% rifampin, 6% tetracycline, 0% trimeprim-sulfamethoxazole, 2% cefamandole, 5% cefaclor, 2% moxalactam, and 0% for the remaining third-generation cephalosporins cefotaxime, ceftazidime, and ceftizoxime. Unlike ampicillin-resistant isolates, 100% of ampicillin-susceptible isolates had relatively low cefaclor MICs of less than or equal to 4 micrograms/mL (mg/L), suggesting a relatively increased H. influenzae beta-lactamase effect on cefaclor in comparison with the other cephalosporins tested.

Asunto(s)

RESUMEN

The bactericidal dynamics of oxacillin against four Staphylococcus aureus isolates with known 24-h "persister" percentages were studied by using the agar dilution plate count method. Isolates were selected to provide a representative spectrum whose individual 24-h trough intrinsic persister percentages ranged from greater than 1 to less than 0.01%. Resultant agar dilution plate count method killing curve patterns were found to be reproducible and served to characterize each isolate. The paradoxical effect was observed for each isolate, with paradoxical peaks tending to develop and diminish sequentially during the course of oxacillin action. The observed strain-dependent dynamics of oxacillin killing underscore the artifactual nature of the so-called tolerance phenomenon and negate the usefulness of MBCs and MBC/MIC ratios for the characterization of S. aureus isolates.

Asunto(s)

RESUMEN

The possible introduction of error into Staphylococcus aureus oxacillin intrinsic persister measurements by the agar dilution plate-count method resulting from technical factor variations associated with inoculum preparation was investigated. pH stability and exponential growth were shown to be present at the time of inoculum standardization. The use of glass or plastic tubes and the contamination of tube walls during inoculum broth culture produced no differences in test results. For S. aureus and oxacillin, the agar dilution plate-count method is a new and reliable approach to the quantitative study of bacterial inhibition and killing.

Asunto(s)

RESUMEN

During a 15-month period, 12,343 consecutive routine urine cultures from female patients were screened for the presence of Gardnerella vaginalis. Of the positive urine cultures, Escherichia coli was found in 1,256 (57%) and presumptive G. vaginalis in 163 (5%). Of the 163 presumptive G. vaginalis isolates, 115 were present in quantitative categories sufficient to suggest the diagnosis of probable urinary tract infection. Of these 115 isolates, 92 were available for specific identification, of which 69 (75%) proved to be G. vaginalis. Comparison of clinical urinary tract diagnoses for the 69 G. vaginalis patients and a matched cohort of E. coli patients showed a significant correlation of E. coli recovery with the diagnosis of urinary tract infection and a significant correlation of G. vaginalis recovery with no urinary tract infection. G. vaginalis was frequently recovered from nonsymptomatic pregnant patients. G. vaginalis appeared at best to be an uncommon urinary tract pathogen.

Asunto(s)

Asunto(s)

RESUMEN

A case of childhood intervertebral diskitis caused by Kingella kingae is presented. In a review of the literature, the authors found 33 reported cases of infection caused by species of the Kingella genus, of which 29 were due to K. kingae. Of the 33 cases, 42% represented bacterial endocarditis and 48% bone and joint infection. Of the 16 bone and joint infections, 11 represented septic arthritis, 3 osteomyelitis, and 2 intervertebral diskitis, the latter finding making the authors' case of K. kingae intervertebral diskitis the third to be reported. A review of the bacteriologic findings in cases of childhood intervertebral diskitis indicates a prominent role for fastidious microorganisms and the need for careful attention to specimen procurement and microbiologic processing.

Asunto(s)

RESUMEN

The influence of technical factor variations on serum bactericidal and serum inhibitory titers was studied by using Staphylococcus aureus clinical isolates versus oxacillin-spiked human serum. Parallel tests, both with and without the use of beta-lactamase in count plates to inactivate oxacillin carryover, were performed with a conventional macrodilution approach, a carefully controlled macrodilution procedure, and a standard microdilution method. Careful control of technical factor variations diminished the incidence of low serum bactericidal titers and decreased the dispersion of results, a finding corollary to the known influence of technical factor variations on the measurement of MBCs. The incorporation of beta-lactamase into count plates resulted in a shift of serum bactericidal titers to lower values. The microdilution method appeared to be least influenced by technical variations and, with the addition of beta-lactamase to count plates, provided the best results.

Asunto(s)

RESUMEN

A total of 398 consecutive clinical staphylococcus isolates, of which 205 were coagulase negative and 193 were coagulase positive, were tested in parallel by using AutoMicrobic system Gram-Positive Susceptibility-MIC cards and modified Mueller-Hinton agar containing 4% NaCl and oxacillin (6 micrograms/ml). The AutoMicrobic system cards correctly detected 103 of 104 (99%) oxacillin-resistant coagulase-negative isolates with no reports of false resistance.

Asunto(s)

RESUMEN

AutoMicrobic system (Vitek Systems, Inc., Hazelwood, Mo.) gram-positive susceptibility-MIC (GPS-MIC) cards and GPS cards were compared for their ability to detect methicillin-resistant Staphylococcus aureus (MRSA) in two groups of isolates. MRSA isolates from the first group were detected at rates of 23 and 85%, and MRSA isolates from the second group were detected at rates of 95 and 88% for GPS and GPS-MIC cards, respectively. Detection of MRSA by the AutoMicrobic system lacks sensitivity and remains unsatisfactory unless accompanied by supplemental testing.

Asunto(s)

RESUMEN

We evaluated the use of mannitol salt agar with oxacillin for use as a primary screening medium for the simultaneous detection and identification of methicillin-resistant Staphylococcus aureus in clinical surveillance specimens. Oxacillin agar dilution susceptibility tests with mannitol salt agar and Mueller-Hinton agar were performed in parallel with disk-agar diffusion testing on 95 oxacillin-susceptible and 105 oxacillin-resistant S. aureus stock isolates. MICs were found to be comparable, showing distinct separation of susceptible and resistant isolates into two groups with MICs of less than or equal to 2 and greater than or equal to 32 micrograms/ml, respectively. In accord with these findings, 4 micrograms of oxacillin per ml was selected for use in the screening medium. For performance evaluation, mannitol salt agar with 4 micrograms of oxacillin per ml was compared with mannitol salt agar without oxacillin by performing parallel screening tests on 153 clinical surveillance specimens. For detection of methicillin-resistant S. aureus, mannitol salt agar with 4 micrograms of oxacillin per ml was as sensitive as mannitol salt agar without oxacillin and required significantly fewer confirmatory tests. For primary identification of methicillin-resistant S. aureus, mannitol salt agar with 4 micrograms of oxacillin per ml was 6.4% false-positive and 1.1% false-negative, with a 93.6% positive predictive value. These findings indicate that mannitol salt agar with 4 micrograms of oxacillin per ml can be used as a reliable and cost-effective screening medium for the simultaneous detection and identification of methicillin-resistant S. aureus in clinical surveillance specimens.

Asunto(s)