Citrobacter freundii mutants deficient in host specificity functions and their recipient ability for foreign deoxyribonucleic acid

Diarrhea-associated biofilm formed by enteroaggregative Escherichia coli and aggregative Citrobacter freundii: a consortium mediated by putative F pili

BACKGROUND

Enteroaggregative Escherichia coli (EAEC) are enteropathogenic strains identified by the aggregative adhesion (AA) pattern that share the capability to form biofilms. Citrobacter freundii is classically considered as an indigenous intestinal species that is sporadically associated with diarrhea.

RESULTS

During an epidemiologic study focusing on infantile diarrhea, aggregative C. freundii (EACF) and EAEC strains were concomitantly recovered from a severe case of mucous diarrhea. Thereby, the occurrence of synergic events involving these strains was investigated. Coinfection of HeLa cells with EACF and EAEC strains showed an 8-fold increase in the overall bacterial adhesion compared with single infections (P < 0.001). The synergic effect was mediated by physical interactions among the bacteria and primed in the absence of chemical signaling and without the participation of host cells. Thus, significant increases (2.7-fold on average) in bacterial adhesion were also observed during the formation of mixed biofilms on abiotic surfaces. Bacterial settling assays showed that EAEC strains harboring F-pili genes (traA) were capable of forming bacterial aggregates only in the presence of EACF. Scanning electronic microscopy analyses revealed that bacterial aggregates as well as enhanced biofilms formed by EACF and traA-positive EAEC were mediated by non-bundle forming, flexible pili. Moreover, mixed biofilms formed by EACF and traA-positive EAEC strains were significantly reduced using nonlethal concentration of zinc, a specific inhibitor of F pili. In addition, EAEC strains isolated from diarrheic children frequently produced single biofilms sensitive to zinc.

CONCLUSIONS

Putative F pili expressed by EAEC strains boosted mixed biofilm formation when in the presence of aggregative C. freundii.

Genetics of colicin E susceptibility in citrobacter freundii

The insensitivity of Citrobacter freundii to the E colicins is based on tolerance to colicin E1 and resistance to colicins E2 and E3. Spontaneous colicin A resistant mutants of C. freundii also lost their colicin E1 receptor function. Sensitivity to colicin E1 can be induced by F'gal+tol+ plasmids, the tol A+ gene product of which is responsible for this effect. Receptor function for colicins E2 and E3 is induced by the E. coli F'14 bfe+ plasmid, which is also able to enhance notably the receptor capacity for colicin E1. The bfe+ gene product of E. coli, which is responsible for these phenomena, also restores the receptor function for colicin A and E1 in colicin A resistant mutants of C. freundii. All results show that there is a remarkable difference between the E. coli bfe+ gene product and the bfe+ gene product of C. freundii and also between the tol A+ gene products of these strains. The sensitivity to phage BF23 parallels the sensitivity to colicins E2 and E3 and is also induced by the F'14 bfe+ plasmid.

A genetic study of tolerance and resistance to colicin A in Citrobacter freundii

Colicin A-insensitive mutants of Citrobacter freundii were isolated and grouped into six phenotypic classes characterized by sensitivity, insensitivity or partial insensitivity to the bacteriocins S6, DF 13 and colicin A, and sensitivity or insensitivity to deoxycholate (DOC) and ampicillin. Mapping by the gradient-of-transmission method revealed the chromosomal regions in which the responsible genes are situated. Res-3 mapped near pur between pur and thr; Tol-5 mapped between aro and ilv and Tol-4 between gal and pur; Tol-1, Tol-2 and Tol-3 are situated close to gal. All the mutations that mapped near gal rendered the bacteria more sensitive to DOC and ampicillin. Complementation analysis with E. coli plasmids showed that the three phenotypic groups that map near gal were complemented by E. coli plasmids and fall into three complementation groups. Two of these are quivalent with the tol A and tol B genes in E. coli.

Behavior of a hybrid F' ts114 lac+, his+ factor (F42-400) in Klebsiella pneumoniae M5a1

Episome F' ts114 lac+, his+ (F42-400) was transferred from Salmonella typhimurium to Klebsiella pneumoniae. From the progeny, a strain of K. pneumoniae able to retransfer the episome was obtained. The His+ phenotype in this strain is temperature sensitive. Escherichia coli female-specific phages phiII, W31, and T3 were shown to plate on K. pneumoniae. From phiII we obtained two derivatives; phiIIK, which plates only on K. pneumoniae, and phiIIE, which plates only on E. coli. Growth of phages T3 and phiIIK was inhibited by F42-400 in K. pneumoniae. Growth in presence of acridine orange in a defined medium at 40 C resulted in a high level of curing. The frequency of His+ cells after growth in acridine orange at 40 C was 0.001%. An extensive search to detect chromosome mobilization by F42-400 in K. pneumoniae, under different experimental conditions, was negative. We cannot exclude the possibility that the low transfer efficiencies prevented our detection of chromosome mobilization. A search among temperature-resistant, acridine orange-curing-resistant, or galactose-resistant derivatives of the K. pneumoniae donor strain failed to reveal any chromosome transfer. Our failure to detect Hfr's may be a result of: (i) the peculiarity of episome F42-400, (ii) the peculiarity of K. pneumoniae chromosome, or (iii) low transfer efficiency. K. pneumoniae-modified F42-400 and phage 424 were restricted by E. Coli K-12. E. coli K-12-modified episome F42-400 and phage 424 were restricted by K. pneumoniae. E. coli C failed to restrict F42-400 modified with K. pneumoniae specificity. The ability of K. pneumoniae to accept F42-400 is less, by about a factor of 50, than that of E. coli C. As an explanation for the differences in the behavior of E. coli C and K. pneumoniae in ability to receive F42-400 it was suggested that recipient bacteria have specific sites for interaction with the F-pilus tip; these are present in E. Coli C, leading to high transfer efficiency, whereas they may not be present (or if present, are not accessible) in K. pneumoniae, leading to low transfer efficiency.