Genetic and physical characterization of trimethoprim resistance plasmids from Shigella sonnei and Shigella flexneri.

Analysis of six Shigella flexneri and four S. sonnei isolates with trimethoprim (Tp) resistance from clinical cases in Ontario has shown that, in all isolates, the Tp resistance is mediated by gene(s) on conjugative, multiple antibiotic-resistance plasmids. The physical and genetic characterization of these plasmids revealed that there are three different Tp resistance plasmids. One group, composed of all six S. flexneri plasmids, consists of plasmids which are about 70 megadaltons (MDa) and inhibit the fertility of an Escherichia coli Hfr strain (Fi+). A representative member of this group, pPT4, demonstrates a weak incompatibility reaction with IncFl plasmid R455-2. Another group, three of the four S. sonnei plasmids, contains plasmids which are about 43 MDa, Fi-, and mediate propagation of phage PRD1. The third group, the remaining S. sonnei plasmid, is 53 MDa, fi+, mediates propagation of phages fd and MS2, and is incompatible with IncFII plasmid R100. These plasmids also have been differentiated by restriction endonuclease fragment profiles. Analysis of pPT4 has revealed that the Tp resistance of this plasmid is transposable. The transposon, Tn536, is different from previously described Tp resistance transposons; it is 16 MDa, and in addition to Tp, it encodes resistance to mercuric chloride ions, spectinomycin, streptomycin, and sulfonamides.

Antibacterial activity of niridazole against Salmonellae.

Fifty-six strains, representing eight species of salmonellae of diverse geographic origin and possessing a variety of antimicrobial resistance profiles, were tested for susceptibility to niridazole by the agar dilution method. Calculated MICs for 50 and 90% of strains were 4.8 and 16.0 mg/liter, respectively, with a susceptibility range of 0.25 to 32 mg/liter. No obvious species differences were noted. Niridazole was found to be rapidly and powerfully bactericidal. No significant difference was detected between MICs and MBCs. Except for a strain-dependent effect with a subset of multiply resistant salmonella isolates, no inoculum effect was demonstrated.

Transfer of plasmids from Escherichia coli to Pseudomonas aeruginosa: characterization of a Pseudomonas aeruginosa mutant with enhanced recipient ability for enterobacterial plasmids.

Pseudomonas aeruginosa strain GT424, derived by the transfer of plasmid R45 to strain GT1, was shown to have an Era+ phenotype (enhanced recipient ability) for enterobacterial plasmids from incompatibility (Inc) groups C, FI, FII, J, N, W, and X. Increase in transfer frequency ranged from 10 to 10(6)-fold. Plasmids of IncFII, IncJ, and IncX were found to be transmissible only to the Era+ strain. Plasmids of Inc groups A-C, M, and T transferred at low frequency (less than 10(-7)) to both GT1 and GT424 and did not respond to the Era+ character of the latter. Also not responsive to the Era character were plasmids R144drd3 (IncI alpha) and RP1 (IncP), which transferred to both GT424 and GT1 at intermediate (10(-4)) and high frequencies (10(-1)), respectively. All plasmid-mediated antibiotic resistances that could be measured as well as UV resistance and susceptibility were found to be expressed in P. aeruginosa. Plasmid-coded phage susceptibilities, however, generally were not phenotypically manifested in the P. aeruginosa strains. The physical structure of plasmids in P. aeruginosa could be classified into four major types. Some of the plasmids underwent additions or deletions and were unstable; the majority, however, appeared to integrate into the chromosome. Some members of IncN, IncC, IncJ, and IncP were found to be transmissible from P. aeruginosa back to Escherichia coli. This is the first report of the successful transfer to P. aeruginosa of plasmids from many of the Inc groups that have been defined in Enterobacteriaceae, namely, FI, I alpha, J, M, T, and X.

Transfer of IncN plasmids to Pseudomonas aeruginosa.

Three of four N plasmids tested were found to be conjugatively transferable from Escherichia coli to Pseudomonas aeruginosa. The plasmids in the Pseudomonas transconjugants differed from the plasmids in the donor E. coli with respect to molecular weight, transfer ability, phenotype conferred, and stability. In some cases, the antibiotic and UV resistance genes appeared to integrate into the P. aeruginosa chromosome.

Transferable urease activity in Providencia stuartii.

Six urea-positive Providencia stuartii strains were tested for transmissible urease determinants. Two strains, when implanted with "helper" conjugative plasmids, were found to be capable of transferring urease genes to Escherichia coli or urea-negative P. stuartii. Recombination of the urease genes with the helper plasmid in P. stuartii was noted in one case. One of the urea-positive P. staurtii strains was found to harbor a conjugative plasmid which mediated both urease activity and ampicillin resistance. This large plasmid (molecular weight, approximately 140 x 10(6)) was transmissible to and stably maintained in E. coli strains. The demonstration of transmissible genes for urease activity in P. stuartii is significant in that it accounts for previous problems associated with classifying urea-positive strains of this species.

Plasmids of incompatibility group P code for the capacity to propagate bacteriophage IKe.

Seven of eight plasmids of incompatibility group P were found to code for the capacity to propagate bacteriophage IKe in Escherichia coli. Six of the seven plasmids allowed propagation of IKe by one bacterial host (RG172) but not by another (RG176); the other plasmid allowed IKe propagation by both hosts. IKe propagation by a number of E. coli K-12 strains was quite variable. IKeh, an extended host range mutant of IKe, was found to plaque specifically on N+ and P+ strains.

Incidence of the H2 group of plasmids in chloramphenicol-sensitive salmonella isolated in 1974 from clinical sources in Ontario.

Chloramphenicol resistance in salmonella obtained from clinical sources in Ontario was previously found to be often mediated by R plasmids of the H2 incompatibility group. In the present study 40 salmonella strains resistant to one or more of kanamycin, streptomycin, sulfonamides, or tetracycline but sensitive to chloramphenicol, were investigated to determine if they contained R plasmids. Self-transmissible plasmids were isolated from 17 of the strains, and 7 of those showed the bacteriophage inhibition and thermosensitive mechanism of transfer characteristic of H2 plasmids. Entry exclusion and incompatibility experiments confiremd their classification. The results demonstrate that in this population of salmonella, R plasmids of the H2 group are prevalent. Experiments with plasmid-specific phages indicate that the plasmids of this sample, which are not in the H2 group, do not belong to any of the F, I, N, P, or W incompatibility groups.

R plasmids of the S incompatibility group belong to the H2 incompatibility group.

Plasmids of the S and H2 incompatibility groups showed thermosensitive transfer and a bacteriophage inhibition phenotype and were incompatible with one another.

Antimicrobial resistance and resistance plasmids in Salmonella from Ontario, Canada.

Collections of 589 human and 204 animal strains of Salmonella isolated in Ontario during the summer of1974 were examined for susceptibility to 12 antimicrobial agents. Many isolates were found to be resistant to both chloramphenicol (12.4% of the human and 38.2% of the animal sample) and ampicillin. The chloramphenicol resistance almost always occurred in strains which were also resistant to ampicillin and was usually due to a self-transmissible plasmid with a resistance pattern of CmKmSmTc (chloramphenicol, kanamycin, streptomycin, and tetracycline) or CmTc. Ampicillin resistance in these strains was mediated by a variety of plasmids with patterns ApSu (ampicillin and sulfa drugs) and ApSmSu, many of which were nonself-transmissible. Ampicillin resistance in chloramphenicol-sensitive strains was transferable from 21% of the strains, and it was associated with resistance patterns which were different from the self-transferable ampicillin patterns from the chloramphenicol-resistance strains.

Incompatibility and surface exclusion properties of H1 and H2 plasmids.

Plasmids of the H incompatibility group showed two types of surface exclusion and incompatibility interactions. Strong incompatibility and surface exclusion were evident between plasmids within the same subgroup, and recombination frequently occurred between these plasmids after antibiotic selection for the presence of two plasmids in the same cell. Weaker interactions were seen between plasmids of the different subgroups, H1 and H2, and recombination was not detected. Incompatibility between H1 and H2 plasmids led preferentially to the loss of the H1 plasmid, irrespective of the order of entry of the plasmids. These data are consistent with the hypothesis that incompatibility is negatively controlled.

Incompatibility and bacteriophage inhibition properties of N-1, a plasmid belonging to the H2 incompatibility group.

N-1, a plasmid isolated from a strain of Shigella flexneri in Japan more than 10 years ago, mediates the phage inhibition phenotype which has recently been found to be characteristic of plasmids of the H2 incompatibility group. Using the criteria of phage inhibition, surface exclusion and incompatibility, the N-1 plasmid is shown to be closely related to H2 plasmids isolated from non-typhoid salmonella and distantly related to H1 plasmids isolated from Salmonella typhi. Plasmids of other incompatibility groups did not show the H2 type of phage inhibition.

Resistance to chloramphenicol and ampicillin of Salmonella typhimurium in Ontario, Canada.

Combined resistance to chloramphenicol and ampicillin in Salmonella typhimurium is appearing in Canada. Five cases of this type of infection have been noted. The isolates are resistant to streptomycin, sulfonamides, and tetracycline, in addition to ampicillin and and chloramphenicol; four isolates are also resistant to kanamycin. In each isolate the determinant for chloramphenicol resistance is linked to a conjugative plasmid. Four of the chloramphenicol plasmids appear to be related in resistance pattern, level of antibiotic resistance mediated, and temperature sensitivity of mating ability. The same four plasmids are associated in the primary isolates of Salmonella with three different kinds of genetic units mediating resistance to ampicillin. In only one of the five strains are the genes for chloramphenicol and ampicillin resistance linked in the same plasmid.

Inhibition of bacteriophage lambda, T1, and T7 development by R plasmids of the H incompatibility group.

R plasmids from chloramphenicol-resistant salmonella from Ontario are shown to belong to the H(2) incompatibility subgroup and to mediate a broad-spectrum, phage inhibition function.

Temperate phages of Clostridium perfringens type C1.

Four phages isolated from carrier strains of Clostridium perfringens type C belong to two classes. The three phages of class I, c1, c3, and c4, and homoimmune and serologically closely related. The phage of class II, c5, is heteroimmune to the class I phages and not related to them serologically. Transduction experiments with several of the phages were negative. Mutants of the indicator strain with surface alterations occurred spontaneously in stock cultures. Electron micrographs show the phages of each class to be distinct yet similar, having polyhedral heads of about the same diameter 55 nm, and long, flexible tails without sheaths or collars. Phages c4 and c5 were characterized for their lysogenic properties. Phage c4 was inducible with mitomycin C. Both c4 and c5 were temperate viruses by the test of stability of their respective lysogens to phage-specific antisera.