An outbreak of infection caused by a strain of Staphylococcus aureus resistant to gentamicin and tobramycin and other antibiotics occurred in two wards in a hospital. Eight patients were colonized, of whom six had clinical infections. Previous administration of gentamicin appeared to predispose the patients to infection with the strain. Restriction of the use of gentamicin and tobramycin is essential to preserve their value in serious infections.
SUMMARY Two isolates of Proteus mirabilis and four of Esch. coli which would not grow on commonly used commercial sensitivity test media are reported. These organisms appear to be resistant to co-trimoxazole, are exacting towards thymidine, and five of six were isolated from patients who had been treated with co-trimoxazole. A method for the detection of these strains is given and their significance in the laboratory and clinical practice discus-ed.In the last few months strains of Enterobacteriaceae have been isolated by a method to be described which are exacting towards thymidine and/or other substances and are resistant to co-trimoxazole. The following gives some of the characteristics of six organisms isolated and of their possible significance.
Materials and MethodsThe media used were Oxoid MacConkey agar, Oxoid diagnostic sensitivity test agar (DST), Wellcotest sensitivity agar (WST, Burroughs Wellcome), and Oxoid Columbia base agar. Defibrinated horse blood, lysed with saponin, was added to a final concentration of 5 % (v/v), where stated in the text. For some months we have been screening all 'coliform' organisms for the failure to growcharacteristic on DST lysed horse blood agar. The technique adopted is as follows.All 'coliform' organisms are grown in 5 ml peptone water for four hours to produce a suspension of faint turbidity. A 10-3 dilution of this growth is made in non-nutrient medium such as buffered saline or Ringer's solution (1 drop from a 50-dropper pipette in 20 ml diluent, or 1,3 mm loop of culture in 5 ml diluent); this is flooded onto the test plate and the excess fluid quickly sucked off. The test plates are DST agar with 5 % lysed horse blood, and it is essential that light inocula are used (of the order of 103 to 105 organisms per ml, 10' organisms being the optimum) which give heavy but not confluent growth. If the inoculum is too heavy 'break through' growth takes place.
o.a.s.i.s. (Unipath Ltd., Basingstoke, United Kingdom) is a new automated blood culture system. The metabolism of microorganisms is detected by measuring changes in the pressure of the headspace of blood culture bottles. These changes are measured by monitoring the position of a flexible sealing septum, every 5 min, with a scanning laser sensor. This noninvasive system can detect both gas absorption and production and does not rely solely on measuring increasing carbon dioxide levels. A research prototype instrument was used to carry out an evaluation of the media, the detection system, and its associated detection algorithm. In simulated blood cultures, o.a.s.i.s. supported growth and detected a range of clinical isolates. Times to positivity were significantly shorter in o.a.s.i.s. than in the BACTEC 460 system. Results of a clinical feasibility study, with a manual blood culture system as a control, confirmed that o.a.s.i.s. was able to support the growth and detection of a variety of clinically significant organisms. On the basis of these findings, full-scale comparative clinical trials of o.a.s.i.s. with other automated blood culture systems are warranted.
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