Interaction Of Colicins - Typically, a third of E. coli strains produces a mitomycin C-inducible bacteriocin (Riley and Gordon 1996). For example, 24% of the E. coli isolates from humans and 33% of the isolates from mammals were colicin producers.
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| Interaction Of Colicins |
Although there have been relatively few representative surveys of other members of the Gram-negative bacteria, similar frequencies of production are observed in these studies (Reeves 1972; Riley et al. 2003). However, the prevalence of colicinogenic strains may vary from 10 to over 70% among different E. coli populations (Riley and Gordon 1996; Gordon et al. 1998). Much of the variation in the frequency of colicinogenic strains in populations of E. coli is undoubtedly due to the dynamic interactions occurring between colicinproducing, resistant and sensitive cells (Riley and Gordon 1999), which result in temporal fluctuations in the relative frequencies of these three phenotypes. Such temporal fluctuations have been observed in vitro (Kerr et al. 2002) and in a population of E. coli isolated from house mice, in which the frequency of colicinogenic isolates declined from 71 to 43% over a 7-month period (Gordon et al. 1998).
Although the frequency of colicinogenic strains in a population is expected to vary naturally, theoretical considerations and empirical observation suggest that the environment in which the cells live also influences the frequency of colicinogenic cells. The predictions of mathematical models indicate that colicin-producing cells will have an advantage in benign habitats (resulting in high population growth rates) whereas harsher habitats will favour nonproducers (Frank 1994). There is some empirical evidence to support this prediction. Population growth rates and cell densities are significantly lower in the external environment than in the lower intestine of mammals and, in Australia, 9% of E. coli isolated from the environment were producers, a significantly lower proportion than the 30% observed for Australian faecal isolates. The frequency of colicin-producing E. coli also depends on the type of host from which the cells are isolated. In Australian mammalian carnivores, the frequency of colicin-producing strains is about 20% whereas colicin production is twice as frequent in strains isolated from herbivorous or omnivorous Australian mammals. For animals of similar body mass, the turnover rate of the carnivore gastro-intestinal tract is significantly faster than that recorded in herbivorous or omnivorous mammals (Hume 1999). The predictions of mathematical models support the observation that the cost of colicin production can result in colicinogenic strains being disadvantaged when living in hosts with high gut turnover rates (unpublished data). There is additional evidence for the importance of the host environment in determining the likelihood that a strain will be colicinogenic. There are two species in the genus Hafnia (Okada and Gordon 2003; Janda et al. 2005). A collection of Hafnia species 2 strains, isolated from fish, reptiles and mammals from across Australia, was screened for the presence of a bacteriocinogenic phenotype. The results of this screening showed that 4% of the isolates from fish, 64% of the isolates from reptiles and 29% of the isolates from mammals were bacteriocin producers.
Faecal isolates of E. coli can be assigned to one of four main genetic groups (subspecies), designated A, B1, B2 and D (Ochman and Selander 1984; Herzer et al. 1990). Strains of the four groups appear to occupy different ecological niches (Gordon and Cowling 2003; Gordon et al. 2005). For E. coli, it is well established that some traits, particularly virulence factors associated with extraintestinal disease, are largely confined to particular genetic groups (Johnson and Stell 2000). Genetic group membership also appears to predict the frequency of colicin production. Among the Australian mammal isolates, 46% of genetic group B2 strains produce a colicin, compared to only 23–27% of strains in the genetic groups A, B1 or D. The reasons for these differences are unknown.
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