The Diversity of Bacteriocins in Gram-Negative Bacteria

Summary

The frequency and diversity of bacteriocin production varies greatly among bacterial populations. The dynamic interactions occurring among bacteriocinproducing, sensitive and resistant cells are likely responsible for much of this variation. However, the frequency of bacteriocinogeny and the diversity of bacteriocins produced are also determined by the habitat in which the population lives and by the genomic background of the producing strains. The production by a cell of two or more bacteriocins is a common phenomenon, at least in Escherichia coli. Further research is required if we are to understand the nature of fitness advantages accruing from multiple bacteriocin production, and how to best exploit bacteriocins as replacements for traditional antibiotics and in the creation and selection of bacterial strains for use as probiotics.

Introduction

Allelopathy is the production of chemical compounds which are toxic to other organisms but not to the producers of these compounds. For microorganisms, there is a wealth of data demonstrating that allelopathy is an important mediator of intra- and inter-specific interactions and consequently, a significant factor in maintaining microbial biodiversity (Chap. 6, this volume). In bacteria, these allelopathic substances include metabolic by-products such as ammonia or hydrogen peroxide; the ‘classical’ antibiotics such as bacitracin and polymyxin B, lysozyme-like bacteriolytic enzymes and the bacteriocins. The bacteriocins produced by Gram-negative bacteria are diverse. Over 30 bacteriocins from Escherichia coli have been identified and, undoubtedly, more have yet to be discovered. The diversity present in other Gram-negative species, including other members of the Enterobacteriaceae, is largely unexplored. The molecular mechanisms by which this diversity has arisen, at least for that class of bacteriocins known as colicins, is well understood and is discussed elsewhere (Chap. 3, this volume). However, the factors influencing the frequency of bacteriocin production and the diversity of bacteriocins in populations of bacteria are largely unknown. Acquiring this knowledge is essential, not only if we are to understand the role bacteriocins play in shaping bacterial communities in natural environments but also because there is an increasing desire to exploit bacteriocins to solve a range of applied problems.

The Frequency of Bacteriocin Production

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. 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). 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.

Microcins

There has been far less work on the frequency of microcin production in E. coli or other Gram-negative bacteria. All the microcins characterised to date are secreted from the cell, rather than being released as a consequence of cell lysis (Braun et al. 2002). It has also been suggested that as much as 90% of the microcin produced by a cell may be retained within the cell (Braun et al. 2002). Consequently, there is no reliable and simple phenotypic method for assaying microcin production. The E. coli isolated from Australian humans and mammals were screened for seven microcins, using a PCR-based approach. Of the human isolates screened, 32% were microcin producers whereas, among the isolates from mammals, 9% were microcin producers. Why microcin production is less common in E. coli isolated from mammals compared to humans is unknown. In the isolates from humans, microcin production is significantly more prevalent among genetic group B2 strains (47%) than among A (16%), B1 (18%) or D strains (9%). The frequency of microcin production in both of these E. coli collections has almost certainly been underestimated, as a PCRscreening approach can be used only for those microcins which have been genetically characterised.

Sumber bacaan: Bacteriocins Ecology and Evolution  [M.A. Riley M.A. Chavan (Eds.)]