The phytopathogenic bacteria are quite ordinary bacteria which cause a variety of diseases in most groups of plants. Some of them can also cause nosocomial infections of compromised patients in hospitals. We are presently working on the physiology and genetics of plant virulence in Erwinina amylovora, the causal agent of fireblight of pears and apples. A genetic transmission system has been developed (and is being improved); using it, we have been able to transfer by conjugation the genetic capacity to infect pears. The outer membrane of the cells of wild-type, virulent E. amylovora is unusually permeable, and this property may have some bearing on its pathogenesis which allegedly stems from formation of a large-molecule polygalactose toxin. Genetic donor systems are being developed also in the other groups of Erwinia: the yellow-pigmented plant E. herbicola group (which also cause diseases in man) and the pectolytic E. carotovora group. Emphasis is being placed on the role of these phytopathogenic bacteria in causing human disease, and the study is being extended to related nonphytopathogen, Serratia, which also causes nosocomial infections--a selective medium for isolating Serratia has been developed and is being used to hunt for Serratia within and without the hospital. Plant and human strains of Erwinia have been run through the usual medical diagnostic tests, with the conclusion that some of them mimic members of other genera when using these conventional procedures. The expression of two sorts of E. coli plasmids (that control hemolysin and enterotoxin production) in Erwinia will soon be investigated, partly in the hope of learning how plant bacteria can "become" human pathogens. Collection of data for a computer-based taxonomic analysis of the phytopathogenic coryneform bacteria is almost completed, as is a study of morphology and morphogenesis in this group. BIBLIOGRAPHIC REFERENCES: Starr, M.P., M. Mandel, and N. Murata. 1975. The phytopathogenic coryneform bacteria in the light of DNA base compositions and DNA-DNA segmental homology. Journal of General and Applied Microbiology, 21, 13-26. Pugashetti, B.K. and M.P. Starr. 1975. Conjugational transfer of genes determining plant virulence in Erwinia amylovora. Journal of Bacteriology, 122, 485-491.