John D. Hopkins

Molecular Epidemiology of Antibiotic Resistance in Salmonella from Animals and Human Beings in the United States

We collected serotyped isolates of salmonella from reference laboratories in the United States, tested their susceptibility to antibiotics, and extracted plasmids from isolates that were resistant to a different combination of antibiotics from each of three serotypes. Restriction-endonuclease digestion showed that within each of the three groups, plasmid molecules from animal and human isolates were often identical or nearly identical. One serotype-plasmid combination appeared to be endemic in cattle in 20 states and infected 26 persons in two states. The human cases, which were not recognizably related except for their common plasmids, appeared to be clustered in time but geographically dispersed, like cases in previous outbreaks spread by food products. These findings suggest that resistance plasmids may be extensively shared between animal and human bacteria, and that spread of multiresistant strains of salmonella among animals and human beings, as observed in Britain, may have been undetected in the United States for lack of comparable surveillance.

A new class of promoter mutations in the lactose operon of Escherichia coli

Abstract The isolation and genetic characterization of a number of mutations that are located in the promoter region of the lac † operon are described. These mutations have reduced levels of lac operon expression in a wild, type ( crp + cya + ) genetic background. Three of the mutations also have lower levels of lac operon expression than lacP + in a crp − cya − genetic background, that is in the absence of the catabolite activator protein and 3′,5′-adenosine cyclic monophosphate. These three mutations are located nearest to the lac operator. They define a second essential site in the promoter region.

A new class of mutants in DNA polymerase I that affects gene transposition

A mutant of Escherichia coli strain K12 is defective in transposition of both the transposons Tn5 and Tn10 and the insertion sequences IS1 and IS5. In addition to the defect in transposition, the mutant is also sensitive to methylmethane sulfonate and ultraviolet light, does not grow phage lambda red and is missing the polymerizing activity and the 5′−3′ exonuclease activity of DNA polymerase I, indicating that the mutation is in the structural gene for this enzyme. We have designated the mutant allele as polA34. All of the properties associated with this mutant cotransduce with a marker known to be linked to polA. Furthermore, revertants of the mutant to methylmethane sulfonate resistance also regain the normal transposition frequencies of Tn5, IS1 and IS5. Complementation tests using the diploid polA34/polA show that the sensitivity to methylmethane sulfonate, and the defect in transposition is recessive to the wild-type. Some revertants of polA34 (called polA34 spa) restore resistance to methylmethane sulfonate and u.v. and partially restore the polymerase and 5′−3′ exonuclease activity but do not restore transposition. Thus we conclude that neither the polymerase activity nor the 5′−3′ exonuclease activity are required in transposition, but rather some other property of DNA polymerase I is needed.

Stimulation of precise excision and recombination by conjugal proficient F′ plasmids

SummaryLarge F plasmids such as F′128 stimulate precise excision of the transposons Tn5 and Tn10 in E. coli K12. This stimulation occurs when the transposons are either on the F′128 plasmid or the bacterial chromosome. Stimulation of precise excision is dependent upon conjugal transfer proficient F′ plasmids. Tra− mutations which are defective in conjugal transfer negate this F′128 plasmid stimulation effect. F′128 traS mutations, which are surface exclusion defective and thus permit matings between male cells, thereby increasing conjugal transfer, increase the F plasmid stimulation effect. When the F′ plasmid is present in a cell with the small plasmid, pRS31, carrying the traS to traZ region of F, stimulation of precise excision is no longer observed. This complementation-like activity by pRS31 is abolished by a Tn5 insertion in the traS gene. Data are presented supporting the notion that F′ plasmid stimulation of precise excision occurs in the recipient during conjugal transfer. F′128 traS also stimulates recA-dependent recombination between DNA sequences on the small, nontransferrable plasmid pRDK41, DNA sequences that are unrelated to those of the F plasmid. The F′ plasmid stimulation of precise excision of Tn5 is not seen with F+ but only with certain Fs with large insertions of chromosomal DNA.

Resistance to Antibiotics in Clinical Isolates of Klebsiella pneumoniae

The antibiotic resistance of Klebsiella pneumoniae isolates from 12 medical centers worldwide, over a 1- to 6-year period, were tested. Clinical isolates of K. pneumoniae were resistant to ampicillin and carbenicillin. Resistance to other antibiotics was less frequent with isolates of K. pneumoniae from 5 of 6 US centers than with those from 6 centers outside the US. In nearly all of the centers, resistance to sulfamethoxazole-trimethoprim, gentamicin, tobramycin, or chloramphenicol was more frequent in isolates of K. pneumoniae than in those of Escherichia coli, while the reverse was true for resistance to tetracycline. Resistance to multiple antibiotics declined gradually in isolates of K. pneumoniae at one center, but rose abruptly again with dissemination of a new plasmid.