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We studied nine solid and two liquid media for their suitability to select Aeromonas and Plesiomonas spp. from human stools, using artificially contaminated samples as well as 254 samples from outpatients with and without diarrhea. Media with optimal sensitivity and specificity for Aeromonas spp. were alkaline peptone-water, Trypticase soy broth with ampicillin, inositol-brilliant green-bile salts agar, dextrin-fuchsin-sulfite agar, xylose-sodium desoxycholate-citrate agar, and Pril-xylose-ampicillin agar. For Plesiomonas sp., alkaline peptone-water and inositol-brilliant green-bile salts agar were optimal. Four strains of Aeromonas spp. were detected in patient samples with these media.

A study of serological, bacteriocine and phage typing of Serratia marcescens was made. Specific 0-antisera of adequate titre were relatively simple to prepare but H-antisera exhibited many heterologous agglutination reactions amongst the type strains. Most of these cross-reactions were not reproduced when immobilization tests with H-sera were performed. Direct haemagglutination tests were used to establish the presence of fimbriae amongst the H-type strains and the results of agglutination tests with non-fimbriate variants of strains indicated that fimbrial antibody in high titre was present in some sera.Replicate typing of 100 pairs of cultures by the phage-typing method indicated that small variations in pattern were common and that larger variations occurred occasionally. Therefore differences in pattern of less than two strong reactions should not be taken as evidence that strains can be distinguished.Cultures of S. marcescens, 273 in total, from six outbreaks of infection in British and European hospitals were typed by 0-serology, H agglutination and immobilization tests, phage typing and bacteriocine susceptibility by a cross-streaking method. The typability of strains by each method was high but the results suggested that no single method was sufficiently discriminating to be used alone for typing. Comparison of the H-type and typing patterns of members of the same 0 serogroup from incidents of infection showed that reliable results were obtained by H-typing or by phage and bacteriocine typing after the application of the appropriate 'difference' rule.The greatest discrimination between strains of the same 0-group was obtained by the use of H-typing or phage typing. UNTRODUCTIONMany different methods have been used for the epidemiological type identification of strains of Serratia marcescens. They include serological typing of 0 and

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• Hospital-associated infections are a problem for patient care. • Hygiene training of staff prevents bacterial contamination of ultrasound probes. • Disinfection of ultrasound probes is an easy method to protect patients.