Hilary Richards

Distribution of Genes for Trimethoprim and Gentamicin Resistance in Bacteria and their Plasmids in a General Hospital

The incidence of trimethoprim resistance in enterobacteria causing infection in a London hospital increased from 5.6% in 1970 to 16% in 1979. The proportion of gentamicin-resistant aerobic Gram-negative bacilli had risen to 6.5% by 1979. During a 5-month period in 1977, during which no epidemic was recognized, all isolates resistant to either trimethoprim, gentamicin, tobramycin or amikacin were studied. The proportion of enterobacteria resistant to both trimethoprim and gentamicin (3.8% of the total) was significantly higher than expected assuming no correlation between acquisition of resistance characters. The resistance was transferable in 23% of trimethoprim-resistant and 76% of gentamicin-resistant strains. Trimethoprim resistance was carried by plasmids of seven different incompatibility groups and in at least four instances was part of a transposon. Gentamicin resistance was determined by plasmids of three groups - IncC, IncFII and IncW. Transposition of gentamicin resistance was not shown, though this may have been the means of evolution of the gentamicin R plasmids of InW, which determined aminoglycoside acetyltransferase, AAC(3). Some bacterial strains with their plasmids were endemic. There was evidence for these plasmids (i) acquiring new resistance genes by transposition, (ii) losing resistance genes by deletion and (iii) being transferred between bacterial species in the hospital.

Plasmids and transposons and their stability and mutability in bacteria isolated during an outbreak of hospital infection.

Abstract In an outbreak of hospital infection caused by Klebsiella aerogenes type K-16 isolates over a 3-month period carried, apparently unaltered, a cryptic 90-Megadalton (Md) plasmid (unclassified) and a multiple-resistance 65-Md plasmid of IncM. The IncM plasmid, identified in environmentally related strains of Citrobacter koseri and Escherichia coli, showed minor variations from that in the klebsiella vector. The IncM plasmids, as well as all wild host strains cured of the IncM plasmids, carried a transposable DNA sequence, encoding trimethoprim and, in every case but one, streptomycin resistance. This transposon appeared identical with Tn7, previously identified in unrelated plasmids in bacteria from different environments.

SALMONELLA TYPHI IN VIVO ACQUIRES RESISTANCE TO BOTH CHLORAMPHENICOL AND CO-TRIMOXAZOLE

Salmonella typhi in the bowel of a patient with enteric fever treated with chloramphenicol and later co-trimoxazole acquired resistance to these drugs. Chloramphenicol and sulphonamide resistances were determined by one plasmid, of IncH, whose carriage also changed the Vi phage type. Trimethoprim resistance was determined by a transposon, Tn7, carried in another plasmid, of IncI. The same resistance genes were identified in a strain of klebsiella isolated from the patient.

R plasmids of a new incompatibility group determine constitutive production of H pili

Abstract R plasmids of a new incompatibility group, IncHII, determined the constitutive production of H pili, had high molecular weights, and determined tellurite resistance. They were designated IncHII because, during incompatibility tests, they sometimes eliminated or were eliminated by, previously described IncH plasmids, which they resembled in several respects. Nevertheless, stable and separate coexistence, i.e., compatibility, with plasmids of IncH1, IncH2, and IncH3 was demonstrated. The latter subgroups, members of which are all incompatible with one another, were distinguished on the basis of DNA-DNA hybridization experiments ( N. D. F. Grindley, G. O. Humphreys, and E. S. Anderson, 1973 , J. Bacteriol. 115, 387–398; A. F. Roussel, and Y. A. Chabbert, 1978 , J. Gen. Microbiol. 104, 269–276.); it is proposed that they be called IncHI, the subgroups being HI1, HI2, and HI3.

Analysis of the site for second-strand initiation during replication of the Streptomyces plasmid pIJ101.

The indigenous plasmid pIJ101 is the parent of many cloning vectors used in Streptomyces. One early pIJ101 derivative, pIJ702, has been particularly widely used. pIJ702 lacks sti:cop/korB and accumulates single-stranded DNA (ssDNA). The 1.2 kb BclI-BclI sti:cop/korB and 0.7 kb SpeI-BclI sti regions were isolated from pIJ101 and cloned into pIJ702 at the PstI site in both orientations. No ssDNA was detected in constructs containing sti present in its correct orientation with respect to the basic replicon, with or without cop/korB. Constructs which contained sti in the reverse orientation did accumulate ssDNA. Thus, sti is only active as the site for second-strand synthesis in its natural orientation. Furthermore, sti inserted in either orientation into the structurally unstable pIJ702-pUC8 shuttle vectors prevented them from rearranging in S. lividans. The sti function was defined to a 0.53 kb SpeI-SacII fragment and the probable site for second-strand initiation (ssi) was identified.

Reclassification of incompatibility group L (IncL) plasmids

Abstract Plasmids classified as IncL are shown to be incompatible with plasmids of IncM and we propose to reclassify them as IncM.

Diversity of plasmids responsible for multiple resistance in Klebsiella serotype K2.

Klebsiella of capsular type K2 were investigated to find out whether a single epidemic clone was the source of many outbreaks of infection in different hospitals, in different areas over a period of five years. The klebsiellas studied were found to be very similar; they were of the same biotype, had similar klebecin sensitivity patterns and carried multiple drug-resistance plasmids; however, characterization of these plasmids showed that they were heterogeneous. Thus there was not a single epidemic bacterial clone.

Plasmids and transposons acquired by Salmonella typhi in man.

Abstract Salmonella typhi, in a patient with enteric fever, acquired two plasmids that together conferred resistance to the drugs used to treat the illness. One of them, of IncHI, conferred chloramphenicol and sulfonamide resistance and altered the phage type of the S. typhi. An indistinguishable plasmid was present in a strain of Klebsiella aerogenes probably carried by the patient since before the start of therapy. The other plasmid, belonging to no known Inc group, carried a trimethoprim-resistance transposon, indistinguishable from Tn7. The klebsiella, believed to have been the source of the chloramphenicol-resistance plasmid, carried the Tn7-like transposon but on a different plasmid from that in the S. typhi. All the resistance genes acquired by the S. typhi could thus have come, directly or indirectly, from the klebsiella. Both R plasmids from the klebsiella, though compatible with one another, determined H-type pili and resistance to potassium tellurite; their interrelationship is discussed.

THE DEVELOPMENT OF PLASMID VECTORS

Publisher Summary The ability to propagate and clone individual segments of DNA by linking them in vitro to a replicon indigenous to a particular host cell has become so central a tool in molecular biology that it is sometimes difficult to remember. Bacterial plasmids have been studied for many years because many of the special characteristics displayed by bacteria of medical, agricultural, industrial and environmental importance are determined by genes carried on plasmids. Studies of these genes opened up the study of the biology of plasmids, their replication, maintenance and transfer functions as well as their evolution by the acquisition of transposons carrying new genes and which can bring about genome rearrangements. pSCl0l is not an ideal plasmid vector as the site for the restriction endonuclease, EcoRI does not inactivate a gene whose loss of function can be assayed. ColE1, another naturally occurring small plasmid, which was also used as a plasmid vector had similar problems. ColEl produces the small bacteriocin colicin E l and also codes for immunity to this protein.

Trimethoprim-Resistant Bacteria in Hospital and in the Community: Spread of Plasmids and Transposons

Trimethoprim is a very effective synthetic antibacterial drug that was introduced for use in human and veterinary medicine about 10 years ago in Europe (7 years in the US). Until recently it has been used always in conjunction with a sulfonamide. Trimethoprim and sulfonamides act synergistically, at different points upon the folic acid cycle of bacteria and using both drugs together should prevent the emergence of resistant mutants1. Trimethoprim-sulfonamide preparations are effective against a wide range of bacteria and have been extensively used in treating urinary, respiratory and, to a lesser extent, gastrointestinal infections in hospitals and in the community.