In this study, the average prevalence of bacteriocinogenic E. coli strains isolated from fecal microflora was 54.4%. This value is close to the upper range limit seen in previous studies, where the prevalence of bacteriocinogenic E. coli strains varied from 25 to 55% [15, 21, 26, 27, 29–31]. However, these studies differed in a number of important ways including cultivation conditions and indicator bacteria used for detection of bacteriocin production and/or in the number of detected bacteriocin genes. Older studies on the prevalence of bacteriocinogeny in fecal E. coli strains only focused on the identification of colicin production [30, 32]. While Šmarda and Obdržálek (2001) used five different indicator strains to detect colicin production in the fecal E. coli strain 1043 , Achtman et al. (1983) used 2 indicator strains for the detection of colicin producers in a sample of 234 fecal E. coli strains . More recently, Gordon and O’Brien (2006) used PCR with 19 bacteriocin genes to screen 266 fecal E. coli strains (38% of which were bacteriocinogenic) , and Šmajs et al. (2010) detected 29 bacteriocin types in 411 fecal E. coli strains (55% of which were bacteriocin-encoding strains) .
Our results have revealed that the frequency of bacteriocinogeny in E. coli strains positively correlates with the detected number of virulence determinants. Bacteriocinogeny increased by as much as 75–80% depending on the number of encoded virulence factors. To our knowledge, this is the first time that the correlation between bacteriocinogeny frequency and the number of encoded virulence factors has been shown. This finding suggests that at least some bacteriocin-encoding genes should be considered as factors which increase the virulence of E. coli strains.
E. coli strains encoding only fimbriae type I did not show differences in the frequency of bacteriocinogeny compared to strains without genes for virulence factors. The role of fimbriae type I in development of human infections is not clear. Although deletion of the fim gene cluster from virulent E. coli strain O1:K1:H7 attenuated virulence in the urinary tract infection (UTI) model ; possession of fimbriae type 1 in E. coli strains from different sources was not found to correlate with the ability to cause UTIs [34–39]. Several virulence factors, typical for diarrhea-associated E. coli strains, including pCVD432 (EAggEC), ial/ipaH (EIEC), eaeA/bfpA (EPEC) and afaI (DAEC) were not found to be associated with bacteriocin genes. Bacteriocin producers therefore appear to be mainly associated with ExPEC virulence factors (E. coli strains containing combinations of sfa, pap, aer, iucC, cnf1, α-hly determinants). The occurrence of these virulence factors were associated with both chromosomally (microcins H47 and M) and plasmid encoded colicin (E1, Ia and S4) and microcin types (B17, V).
Presently, several bacteriocins including colicin E1, and microcins H47, I47, E492, M, and V are considered virulence factors in extraintestinal pathogenic E. coli strains [20–23]. Azpiroz et al. and Budič et al. found an association between production of microcins H47, I47, E492, M, and V and the distribution of virulence factors (i.e. hlyA, cnf1, usp, iroN, iroCD, fyuA, papC, papG and tcpC) in uropathogenic strains of E. coli; from these results they assumed that production of these bacteriocin types could contribute to development of bacteraemia. Although different sets of virulence determinants and bacteriocin genes were used in these studies, our findings match with these observations.
We also found these associations between bacteriocin production and ExPEC virulence determinants among human fecal E. coli isolates. Moreover, we have found new associations between 3 bacteriocin types (microcin B17, colicins Ia and S4) and the ExPEC virulence characteristics of human fecal E. coli strains. Given that colicin Ia and microcin B17 are known to be encoded on relatively large plasmids, it is possible that the association with more virulent strains is due to other genes being harbored on these plasmids, and not by colicin synthesis itself. However, colicin S4 was found to be encoded on a small plasmid (7.4 kb)  that was similar to the colicin E1-encoding plasmid (6 kb) . Since these small plasmids do not encode virulence factors, colicin S4 appears to be a potentially important virulence factor and/or an important factor of resident E. coli strains.
The presence of virulence determinants (e.g. genes encoding P-fimbriae, siderophore aerobactin, hemolysin and expression of O antigens, which are typical for ExPEC strains; and capsular types K1 and K5) are associated with resident E. coli strains [41–44]. At the same time, ExPEC strains causing extraintestinal infections like urinary tract infections and sepsis/meningitidis are believed to originate from the gut microflora. Their virulence determinants including adhesins (P-fimbriae, S-fimbriae), toxins (e.g. hemolysin, cytotoxic necrotizing factor) and siderophores (e.g. aerobactin) appear to be important for E. coli strains to survive in the extraintestinal environment [45–47].
On the other hand, we found that diarrhea-associated strains from our set of 1181 fecal E. coli had a lower prevalence of bacteriocinogeny and a lower frequency of several bacteriocin producers. In addition, no specific bacteriocin types appear to be associated with virulence determinants that are typical for these strains. Unlike fecal strains which have the characteristics of ExPEC strains, diarrhea-associated strains are not considered to be resident human E. coli strains, which may explain the lower prevalence of bacteriocin genes.
In summary, bacteriocin synthesis is linked to strains with ExPEC characteristics and appears to increase the ability of E. coli to reside in the human gut. Moreover, at least several bacteriocin-encoding genes should be also considered as factors which increase the virulence of ExPEC strains.