A comparison of the performance of commercially available chromogenic agars for the isolation and presumptive identification of organisms from urine.

AIMS: To compare four media-UTI medium, BBL CHROMagar, CPS ID2, and Harlequin CLED-using a collection of fully characterised organisms and subsequent "field trial". METHODS: Seven hundred and eighty seven fully characterised isolates (730 Gram negative bacteria, 47 Gram positive bacteria, and 10 yeasts) were used to test for accuracy of organism identification. To assess isolation rates and ability to detect mixed cultures, 1435 urine samples were cultured in the three best performing chromogenic media (UTI medium, BBL CHROMagar, and CPS ID2) and CLED. RESULTS: The chromogenic agars differed in their accuracy of identification, with BBL CHROMagar performing best and Harlequin CLED performing least well. Similarly, BBL CHROMagar achieved a higher overall isolation rate than UTI medium and CPS ID2. When mixed growth was defined as greater than two organism types, BBL CHROMagar detected more mixed cultures than did UTI medium and CPS ID2, although the differences were not significant. When mixed growth was defined as greater than one organism type the increased number of mixed growths detected by BBL CHROMagar became significant, largely because of differences in enterococcal isolation rates. CONCLUSION: The use of BBL CHROMagar, UTI medium, or CPS ID2 chromogenic agar as a replacement for CLED agar would improve the detection rate of contaminated urine samples. Enhanced identification helps to distinguish different species, facilitating the monitoring of bacterial resistance in support of the national antibiotic strategy. BBL CHROMagar gave the highest overall organism recovery rates, greatest ability to detect mixed cultures, and the most accurate identification of organisms.

Comparison of the performance of selenite cystine and mannitol selenite enrichment broths in the isolation of Salmonella spp. from faeces.

Over a four-month period, 4,658 routine faecal samples were examined in four laboratories and the isolation rates of Salmonella spp. from mannitol selenite (MS) and selenite cystine (SC) broths plated to xylose lysine desoxycholate agar (XLD) compared. The isolation rate by MS was 1.55% and by SC was 1.48%, a small difference which is not statistically significant. Significantly fewer colonies were selected for supplementary testing from SC than MS (p = 0.029), thus reducing confirmatory work. In laboratories where SC is already used for food and environmental work, an opportunity exists to limit stocked salmonella enrichment broths to SC alone.

A comparison of the performance of cystine lactose electrolyte deficient (CLED) agar with Oxoid chromogenic urinary tract infection (CUTI) medium for the isolation and presumptive identification of organisms from urine.

AIMS: As part of the UK antimicrobial resistance strategy and action plan, the Public Health Laboratory Service (PHLS) is required to collect antibiotic susceptibility data so that resistance trends and patterns can be monitored. Most laboratories report urine Gram negative isolates, as "coliforms" according to morphological appearance, but without an acceptable identification system the antimicrobial surveillance data will be meaningless. Commercially available identification systems tend to be expensive and time consuming. Chromogenic agars, which claim to improve the detection of mixed cultures and identification of organisms from urine, have now become available and may provide a cost effective alternative. The primary aim of this study was to compare the performance of cystine lactose electrolyte deficient (CLED) agar with a chromogenic agar (Oxoid urinary tract infection medium; CUTI) in terms of isolation rates and ability to detect mixed cultures. Secondary aims were to evaluate the correlation of "presumptive" identification of isolates from chromogenic media with that of two commercial identification systems and to appraise the sensitivity of the semiquantitative loop and filter paper strip culture techniques. METHOD: One thousand, four hundred and sixty six urine samples were examined in four laboratories using the semiquantitative culture methods of 1 microl loop and filter paper strip. The degree of accuracy of organism identification was measured by comparing the presumptive identification using colony colour supplemented with simple bench tests, with identification obtained from two more complex commercial systems. RESULTS: There was no significant difference between the performance of the loop and filter paper strip methods on the CLED agar, but the CUTI agar performed significantly better than the CLED agar for the detection of significant isolates and mixed cultures. This difference was greater using the loop method. Identification of the organisms using the commercial systems gave > 99% agreement and was therefore considered suitable as a standard against which to compare the presumptive CUTI identification. Using the manufacturer's colony colour criteria in combination with a bench indole test, the CUTI medium was 99% specific for Escherichia coli, although this was reduced to 97% if the indole test was omitted. Citrobacter spp were the most commonly misidentified organisms, giving false presumptive identification as E coli. By testing oxidase activity to differentiate Pseudomonas spp and the absence of indole production to support the identification of Proteus mirabilis, the CUTI medium provided a suitable identification for 86.8% of Gram negative isolates. The remaining 13.2% would require further identification. CONCLUSION: CUTI medium improves the detection of mixed cultures, thereby improving the reliability of reporting of significant isolates when compared with CLED agar. When supplemented with simple bench tests it provides an identification system capable of speciating 86.8% of Gram negative isolates and providing a valuable cost effective mechanism for antimicrobial resistance surveillance.

An evaluation of the performance of XLD, DCA, MLCB, and ABC agars as direct plating media for the isolation of Salmonella enterica from faeces.

AIMS: To compare the performance of four media, singly and in combination, as direct plating media for the isolation of Salmonella enterica from human faeces. METHODS: Two thousand four hundred and nine routine, faecal samples received by four laboratories were inoculated on to xylose lysine desoxycholate (XLD), desoxycholate citrate (DCA), mannitol lysine crystal violet brilliant green (MLCB), and alpha-beta chromogenic (ABC) agars using standardised protocols, reagents, and data collection. Isolates of presumptive salmonellae were identified using standard laboratory techniques and the results were analysed statistically. RESULTS: Direct plating recovered 46 of the 60 possible isolates of Salmonella spp recovered via enrichment broth. No isolates were recovered from direct plating that were not recovered via selenite enrichment. MLCB gave the highest isolation rate individually (84.8%) and amounts of competing flora (CF) did not affect the recognition of colonies. ABC proved highly specific, but insensitive, and isolation rates were adversely affected by any amount of CF. Isolation rates from XLD and DCA were only affected when the CF load was heavy. DCA was least specific, with only 9.01% of picks positive and greatest number of confirmatory tests. XLD and MLCB, in combination, gave the highest isolation rate. CONCLUSIONS: Where the earlier results of direct plating may be advantageous, XLD and MLCB provide the optimal combination. For non-typhi salmonellae, MLCB is the best, single direct plating medium. For routine diagnostic work, XLD is most effective.

Comparison of the performance of lactose and mannitol selenite enriched broths, subcultured to DCA and XLD agars, in the isolation of Salmonella spp. from faeces.

Selenite-based enrichment broths using either lactose or mannitol as a carbohydrate source are generally used as selective enrichment media for the isolation of Salmonella spp. from human faeces in the UK, but few studies have compared the relative efficacy of the available formulations. A variety of solid media is used for the routine subculture from these selective broths, but similarly we have been unable to find published evidence as to which medium performs best. Four thousand and nineteen faecal samples were examined in four laboratories and the isolation rates of Salmonella spp. from lactose (LS) or mannitol selenite (MS) broths, plated onto either xylose lysine desoxycholate agar (XLD) or desoxycholate citrate agar (DCA) were compared. MS performed significantly better than LS (p = 0.02), recovering 95 salmonellae compared with 87. No significant difference in isolation rates was found between XLD and DCA, although colonial appearances of suspected salmonellae on XLD were much more specific, resulting in significantly fewer colonies having to be selected for supplementary testing (p < 0.001) and so reducing confirmatory work. An opportunity exists to simplify holdings of media by choosing to use the MS/XLD combination.

A model-based approach to predicting BOD5 in settled sewage.

Rapid, accurate and reliable measurements of BOD are a very desirable basis for monitoring and controlling wastewater treatment works. Unfortunately, however, producing satisfactory measurements using hardware instrumentation has proved difficult. This paper addresses the issue of BOD estimation using a model-based approach. Two models are developed from historical data using neural network methods. The first model estimates the five-day BOD of the settled sewage using flow, solids, chloride and ammonia data and produces accurate predictions. The second model uses only flow and solids data as inputs yet still produces acceptable (although less accurate) predictions. It was concluded from this that satisfactory estimates of five day BOD can be produced using information that is relatively common to most works. The models are straightforward to apply on-line and offer a method of estimating five day BOD in real-time that is likely to be cheaper, more reliable and easier to maintain than hardware instrumentation.

Antibiotic resistance and serotypes of Streptococcus pneumoniae at Birmingham Public Health Laboratory, 1989-94.

Antibiotic resistance of 1515 consecutive laboratory isolates of Streptococcus pneumoniae between 1989 and 1994 was analyzed. Overall, 39 (2.6%) isolates were resistant to penicillin, 102 (6.7%) resistant to erythromycin and 52 (3.4%) resistant to tetracycline. There was a higher proportion of penicillin resistant isolates from sterile sites compared with "non-sterile sites" (5% vs. 2.2%; P < 0.02). This same pattern occurred with erythromycin (12.5% vs. 5.6%; P < 0.001). From 1989-90 to 1993-94 the penicillin resistance rate increased from 0.8% to 8% and the erythromycin from 5.7% to 8.4%, whereas the tetracycline resistance rate fell from 3.7% to 2.8%. The increase in resistance to penicillin largely occurred in the final 12 months of this study period. One hundred and fifty isolates (9.9%) were serotyped, including isolates from sterile sites and those with penicillin resistance. The commonest serotypes of penicillin-sensitive pneumococci were 14, 19, 9 and 6. The majority of penicillin-resistant pneumococci (PRP) were of serotype 9 (64%) followed by 6, 23 and 19. Overall 95% of these isolates were of serotypes represented in the 23-valent pneumococcal polysaccharide vaccine (Pneumovax II). PRP were more likely to have resistance to erythromycin (23%) or tetracycline (23%) compared to penicillin-sensitive pneumococci (6% and 3% respectively). Most of the PRP were isolated from patients aged over 50 years including 11 isolates from blood cultures of patients with pneumonia or septicaemia. There was a possible epidemiological association between four patients with PRP but surveillance cultures of hospital contacts revealed no extra cases. These results show a worrying increase in infections due to PRP which has implications for clinical and laboratory staff in the diagnosis and treatment of serious pneumococcal infections.

Rapid impedance detection of salmonellas in confectionery using modified LICNR broth.

A commercially available broth, with the addition of inhibitors, was used for the rapid impediometric detection of salmonellas in confectionery. Pre-enrichment in skimmed milk was followed by lysine-iron-cystine-neutral red broth in a Bactometer 123 system. Results were obtained 3 d earlier than is possible with conventional microbiological tests. Some false positives were obtained predominantly with Citrobacter freundii, but this problem is also frequently encountered with traditional methods. Organisms responsible for false positives may be isolated and identified more rapidly than is possible by conventional methods.

Nonlinear response of low-background extrinsic silicon detectors. 2: A revised model.

A revised model is described for computing the nonlinear response of low-background Si:X photoconductive detectors to a given flux variation. The model gives results in good agreement with measured responses to large-amplitude sine wave and square wave flux modulations and implies a simple measurement procedure for calibrating a detector's nonlinear response.