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This work studies the classification, by numerical procedure, of 122 strains belonging or related to the genus Klebsiella. Four classes can be defined: J (= K. pneumoniae: 29 strains for the most isolated from human faeces), K (related to K. pneumoniae and to K. oxytoca: 24 strains isolated from water), L (related to K. pneumoniae although, tetrathionate-reductase+, m-hydrobenzoate+: 37 strains isolated from water or soil), M (= K. oxytoca: 22 strains for the most isolated from water). The importance of the origins of the strains is emphasized.

Forty-three strains of E. aerogenes isolated chiefly in Morocco and France have been studied. Thirty-five strains (81%) are surrounded with a thin capsule, antigenically related to Klebsiella capsular antigens: K4 (2 strains), K4, 59 (1 strain), K11 (2 strains), K26 (7 strains), K42 (5 strains), K59 (3 strains), K68 (14 strains). One strain is capsulated but not typable with Klebsiella capsular antisera. E. aerogenes and Klebsiella capsular antigens are not identical but share common fractions yielding cross reactions. To differenciate E. aerogenes from K. pneumoniae in addition with the three major characters, i.e. motility, ornithine-decarboxylase and urease, the author points out the value of growth in metahydroxybenzoate as sole source of carbon and energy (positive test with E. aerogenes and negative with K. pneumoniae).

A DNA/DNA hybridization study was carried out with 13 strains of Enterobacter gergoviae and 123 strains of Enterobacteriaceae, Mostly Enterobacter and Klebsiella species. E. gergoviae strains were very highly related (relative binding ratios: 88-96%) to the type strain CIP 76-01, as previously reported by Brenner and co-workers. No close genomic relationship was found with other species of Enterobacter and Klebsiella (mean relative binding ratio: 39%) and other Enterobacteriaceae.

A DNA-DNA hybridization study was carried out to determine the taxonomic position of a new group of enterobacteria (group H2) previously studied by numerical taxonomy. All the strains of this group revealed relatively high reassociation binding ratios with the centrotype; 82% of the strains of the group showed more than 69% of reassociation with the centrotype. In spite of numerical taxonomy conclusions, there was no genetic relationship with the species Enterobacter cloacae (higher reassociation binding ratio: 37%). No significant genetic relationship with the other groups of enterobacteria was found. Rahnella aquatilis was defined from phenotypic and genetic data. The strain 133 (CIP 78-65) is proposed as type strain of the species.

A DNA-DNA hybridization study (nitrocellulose filter method) was carried out with 13 strains of Enterobacter sakazakii and 38 strains belonging to other Enterobacter species (E. cloacae, E. amnigenus, E. intermedium and E. gergoviae). E. sakazakii strains were highly related (mean relative binding ratio +/- standard deviation: 89% +/- 10) to the strain R 16-76 (labelled DNA). No close relationship was found with the other Enterobacter species. The low relative binding ratios were not due to a difference in genome size (as shown by reciprocal binding experiments).

Thirty strains of Enterobacteriaceae constitute a new group related to E. aerogenes with the following differential biochemical characters: urease+, KCN-, sorbitol-, mucate- and gelatinase-. Ten of these strains with antigenic similarities and polyresistance to antibiotics were isolated from urine in the same hospital.