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Pathogenicity and Medical Microbiology |
Department of Microbiology, Ume
University, Ume S-90187, Sweden1
Author for correspondence: Xin-He Lai. Tel: +46 90 785 6735. Fax: +46 90 772630. e-mail: Lai.X-H{at}micro.umu.se
The standard Escherichia coli collection of reference (ECOR) strains was examined for ability to exert cytotoxicity towards mammalian cells. A group of strains with functional haemolysin expression caused strong cytotoxicity and detachment in J774 macrophage cells as measured by lactate dehydrogenase release and as observed under a microscope. The expression of haemolysin was monitored by using antisera recognizing the E. coli 
-haemolysin, the HlyA protein, and by quantitative haemolysis assays. The presence of the hlyA gene, which may be part of a pathogenicity island, was also confirmed. These analyses revealed that different ECOR strains express quantitatively different levels of haemolysin. One putative enteroaggregative E. coli (EAEC) strain was also found in the ECOR collection. The EAEC strain was characterized by the clump formation assay, PCR amplification of the EAEC DNA probe sequence and confirmative sequence analysis of the amplified fragment. The EAEC heat-stable enterotoxin 1 gene, astA, was found in 14% (10/72) of the ECOR strains and a consensus sequence for astA was proposed by comparing these sequences with those from pathogens. The astA gene appeared to be plasmid-located. Based on evidence from the work of other laboratories and from the present findings, it is concluded that the ECOR collection contains strains that may represent pathogenic E. coli. It is noted that caution is necessary when handling or disposing of those potentially pathogenic ECOR strains.
Keywords: cytotoxicity, haemolysin, enteroaggregative Escherichia coli, enteroaggregative heat-stable enterotoxin, ECOR
Abbreviations: AA, aggregative adherence; DAEC, diffusely adherent E. coli; ECOR, E. coli collection of reference strains; EAEC, enteroaggregative E. coli; EAST1, EAEC heat-stable enterotoxin 1; EHEC, enterohaemorrhagic E. coli; EIEC, enteroinvasive E. coli; EPEC, enteropathogenic E. coli; ETEC, enterotoxigenic E. coli; LDH, lactate dehydrogenase; MLEE, multilocus enzyme electrophoresis; RBC, horse erythrocyte
The GenBank accession numbers for the sequences reported in this paper are AF159702 and AF160993–161002.
a Permanent address: Priority Laboratory of Molecular Medical Bacteriology of Ministry of Public Health, Institute of Epidemiology and Microbiology, Chinese Academy of Preventive Medicine, Beijing, China.
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