Evaluation of a new selective medium for methicillin-resistant Staphyloccocus aureus.

The aims of this study were to evaluate the performance of a new medium, desferrioxamine oxacillin tellurite egg-yolk mannitol salt agar (DOTEMSA) in detecting methicillin-resistant Staphylococcus aureus (MRSA) and then to compare this medium against the Public Health Laboratory Service (PHLS) recommendation of mannitol salt agar (Oxoid) with oxacillin (OMSA) and Baird-Parker medium with ciprofloxacin (BPC) for the isolation of MRSA. The individual selective agents contained in DOTEMSA were tested against isolates of coagulase-negative staphylococci (CNS) and the medium with all constituents was challenged with various bacteria. Routine screening specimens were plated out on OMSA, BPC and DOTEMSA and the plates were incubated and examined at 24 and 48 h. Tellurite, desferrioxamine and oxacillin each inhibited the majority of CNS isolates; only three (of 103) grew in the presence of all three agents. Sixty-two of 63 isolates of MRSA grew on DOTEMSA and 59 produced lipase. Most other bacteria were inhibited. In all, 184 MRSA isolates were isolated from 540 screening specimens. The sensitivity of OMSA, BPC and DOTEMSA was 42%, 81% and 51% at 24 h, and 60%, 89% and 89% at 48 h. At 48 h, the combination of BPC and DOTEMSA detected 99% of MRSA isolates. Seventy, 49 and one non-MRSA isolates needed investigation for each of the three media respectively. A proposed strategy for MRSA screening would use BPC and DOTEMSA, examining BPC at 24 h and both media at 48 h. Provisional reports could then be issued at 24 h on the basis of rapid agglutination tests to confirm isolates as S. aureus from BPC and at 48 h on the basis of typical colonies from DOTEMSA.

Methicillin-resistant Staphyloccocus aureus: evaluation of five selective media.

This study evaluates the performance in isolating methicillin-resistant Staphylococcus aureus (MRSA) of three media: the reduced salt formulation of mannitol salt agar plus oxacillin (MMSAO); CHROMagar Staph aureus plus ciprofloxacin (CHRAC); and Halifax MRSA medium (HMO), against the previously recommended mannitol salt agar (7% salt) plus oxacillin (OMSAO) and Baird-Parker medium plus ciprofloxacin (BPC). MRSA screening swabs were plated out onto the five selective media and the plates examined at 24 and 48 h. Suspected colonies were confirmed as MRSA by detection of heat-labile DNase, coagulase and/or protein A, and by confirming resistance to methicillin. Of 719 specimens examined, 191 grew MRSA on at least one medium. The relative sensitivities of the five media at 48 h were as follows: BPC, 94%; CHRAC, 70%; OMSAO, 61%; HMO, 56%; and MMSAO, 46%. In addition, BPC gave the least number of unnecessary investigations for non-MRSA isolates. The current advantage of BPC when performing direct culture for MRSA was confirmed. The other ciprofloxacin-containing medium also produced reasonable results. Of the two mannitol salt agar media, the formulation with 7% salt gave better results. HMO proved unreliable at isolating MRSA.

Antimicrobial associations of an outbreak of diarrhoea due to Clostridium difficile.

An increased incidence of diarrhoea due to Clostridium difficile (CDD) at the Northern General Hospital, Sheffield, prompted an investigation into antibiotic use on the renal, medical and geriatric wards, those mostly affected. For the first half of 1997 affected patients on these wards were identified and data collected as to which antimicrobials they had taken between admission and diagnosis. Rates were then calculated of the number of affected patients on a drug over the quantity prescribed (N/1000 x defined daily doses). These were expressed as rate ratios (RR) over the rate of ampicillin/amoxycillin. A corrective procedure was also applied to the RR in an attempt to compensate for bias from the effect of drug combinations. Additionally, quarterly data were collected of numbers of patients with CDD and the amount of cefotaxime, ceftriaxone and cefuroxime issued between January 1996 and September 1997. Those drugs with the greatest association with CDD were cefotaxime (RR = 27.5, P < 0.000001), ceftriaxone (15.1, 0.00004), cefuroxime (8.6, < 0.000001) and ceftazidime (6.4, 0.00008). Several other drugs had increased uncorrected RR (P < 0.05). However, after correction for drug combinations, only the above four drugs remained with significantly increased RRs of 16.9, 8.6, 5.3 and 4.8, respectively. The time course of the outbreak showed a correlation with the use of cefotaxime and ceftriaxone. The incidence peaked in the first months of 1997, following greatest use of cefotaxime in the last quarter of 1996 and of ceftriaxone in the first quarter of 1997. This study confirms the association of cephalosporin and particularly cefotaxime usage with CDD incidence and shows the need to review the use of these drugs, especially in the treatment of respiratory infection. The method is useful for looking at local effects of prescribing with a view to controlling outbreaks of antibiotic resistant bacteria.

Reference laboratory surveillance of syphilis in England and Wales, 1994 to 1996.

Routine surveillance of syphilis of public health importance (infectious, congenital, and neurosyphilis) began in England and Wales in 1994, using reports from six PHLS laboratories that undertook serological and other reference work. One hundred and thirty-one cases were reported in the first two years, including 100 cases of infectious syphilis, with all regions reporting some infectious syphilis. Reports from PHLS laboratories represented one sixth of the number of cases seen in genitourinary clinics (KC60 data), but both systems produced comparable results. Laboratory reports provided more data on risk factors, which were not available elsewhere. This study documents the risk in England and Wales from infections originating in eastern Europe, where sexually transmitted infections including syphilis have reached epidemic proportions. Forty-five per cent of cases of infectious syphilis were reported to have been acquired in the United Kingdom (UK) and 59% of people with infectious syphilis were reported to have been born in the UK. Twenty per cent of the infectious cases were associated directly or indirectly with transmission in Russia or elsewhere in eastern Europe. The majority of infectious cases were from the white ethnic group. Eighty-five per cent of cases of infectious syphilis were reported to have been acquired heterosexually; 26% of male cases were reported to have been acquired homosexually. The PHLS laboratory reporting system is now well established, and could usefully be expanded to include other, non PHLS, laboratories that undertake reference work. It has the capacity to detect changes in the national epidemiology of syphilis, including imported infections.

Comparison of methods for the isolation of methicillin resistant Staphylococcus aureus.

The control of methicillin resistant Staphylococcus aureus (MRSA) relies on the rapid and sensitive detection of carriage. The roles of an enrichment broth, duration of incubation, and Baird-Parker medium containing ciprofloxacin (BPC) were evaluated in comparison with standard media in a centre where the prevalence of ciprofloxacin resistance among MRSA is over 98%. Screening swabs from 402 sites were plated onto BPC, mannitol salt agar (MSA), and MSA with methicillin (MMSA). The swabs were enriched in Tryptone-T broth with 6% salt for 24 hours and the broths subcultured onto BPC, MSA, and MMSA. MRSA was isolated from 134 swabs. Significantly more isolates were obtained by incubating culture plates for 42 hours rather than 18 hours, by the use of broth enrichment, and by addition of methicillin or ciprofloxacin to media. BPC was the most sensitive medium (107 isolates (80%) by direct culture at 42 hours), grew the fewest contaminants, and allowed provisional reporting of 73% of isolates at 18 hours by colonial appearance and use of Staphaurex Plus rapid latex reagent. This may allow the introduction of infection control measures a day earlier than when other established methods are used.

Low incidence of primary infection with toxoplasma among women in Sheffield: a seroconversion study.

OBJECTIVE: To determine the incidence of primary toxoplasma infection in women of childbearing age. DESIGN: A retrospective study of seroconversion on stored pairs of sera taken for routine antenatal testing. SETTING: The antenatal clinics and general practices of Sheffield between July 1989 and October 1992. SUBJECTS: One thousand six hundred and twenty-one women on whom pairs of sera, separated by more than 500 days, had been received. MAIN OUTCOME MEASURE: Rate of primary infection determined by seroconversion as measured by the presence of antibody in the second serum and its absence in the first. RESULTS: Of 1621 women, 160 (9.9%) were initially seropositive. One seroconversion was detected in 2966 woman years of observation for susceptibles. The projected rate for primary infections was 0.23 (95% CI 0.0059-1.3) per 1000 pregnancies. CONCLUSIONS: The rate is much lower than for earlier estimates of incidence in the UK but agrees with a more recent estimate from Sheffield and suggests a falling incidence.

Use of tellurite for the selection of verocytotoxigenic Escherichia coli O157.

Potassium tellurite was assessed for the selection of verocytotoxigenic (VT+) Escherichia coli O157. MICs were higher for VT+ E. coli O157 than for other strains of E. coli and for Aeromonas spp. MacConkey medium containing sorbitol, tellurite and cefixime (TC-SMAC) permitted the growth of VT+ E. coli O157 and Shigella sonnei but partially or completely inhibited the growth of 67% of other strains of E. coli and all or most strains of other sorbitol-non-fermenting species tested. Of 391 rectal swabs from cattle screened on TC-SMAC medium, 26 yielded isolates of VT+ E. coli O157 whereas sorbitol-MacConkey medium with cefixime and rhamnose yielded only nine isolates. Inclusion of potassium tellurite in sorbitol-MacConkey agar markedly increased the rate of isolation of VT+ E. coli O157 from cattle rectal swabs and may do so for other types of specimen.

An improved selective medium for the isolation of Escherichia coli O157.

Sorbitol-MacConkey medium has become widely used for the isolation of verotoxigenic (VT+) Escherichia coli O157. However, many organisms other than VT+ E. coli O157, especially other serogroups of E. coli and Proteus spp., may not ferment sorbitol, and thus may be confused initially with VT+ E. coli O157. Rhamnose is not fermented by VT+ E. coli O157, but is by most sorbitol non-fermenting E. coli of other serogroups. Cefixime is a cephalosporin antibiotic that is more active against Proteus spp. than against E. coli. Inclusion of rhamnose and cefixime in sorbitol-MacConkey agar improves its selectivity for the isolation of VT+ E. coli O157.

Methods for the routine characterisation of isolates of Haemophilus.

Methods and their evaluation are described for the routine characterisation of Haemophilus spp isolates using a test for dependence on V factor and a test for the conversion of delta-amino-laevulinic acid (ALA) to porphyrin in which the ALA is incorporated into a solid medium. A method is also described whereby the difference in the size of the inhibition zones around discs of ampicillin and of amoxycillin/clavulanate can be used to detect the production of beta-lactamase.