A restriction map of Bacillus subtilis tetracycline-resistance plasmid pNS1981

Plasmid-Mediated Antimicrobial Resistance in Staphylococci and Other Firmicutes

In staphylococci and other Firmicutes, resistance to numerous classes of antimicrobial agents, which are commonly used in human and veterinary medicine, is mediated by genes that are associated with mobile genetic elements. The gene products of some of these antimicrobial resistance genes confer resistance to only specific members of a certain class of antimicrobial agents, whereas others confer resistance to the entire class or even to members of different classes of antimicrobial agents. The resistance mechanisms specified by the resistance genes fall into any of three major categories: active efflux, enzymatic inactivation, and modification/replacement/protection of the target sites of the antimicrobial agents. Among the mobile genetic elements that carry such resistance genes, plasmids play an important role as carriers of primarily plasmid-borne resistance genes, but also as vectors for nonconjugative and conjugative transposons that harbor resistance genes. Plasmids can be exchanged by horizontal gene transfer between members of the same species but also between bacteria belonging to different species and genera. Plasmids are highly flexible elements, and various mechanisms exist by which plasmids can recombine, form cointegrates, or become integrated in part or in toto into the chromosomal DNA or into other plasmids. As such, plasmids play a key role in the dissemination of antimicrobial resistance genes within the gene pool to which staphylococci and other Firmicutes have access. This chapter is intended to provide an overview of the current knowledge of plasmid-mediated antimicrobial resistance in staphylococci and other Firmicutes.

A limited number of Bacillus subtilis strains carry a tetracycline-resistance determinant at a site close to the origin of replication

Several strains of Bacillus subtilis, e.g., 168 derivatives and R, were found to carry a single copy of a tetracycline-resistance (TcR) determinant (named tetBS908) at a site close to the origin of replication on the chromosome. This gene is highly homologous (80% identical) to the TcR determinant of plasmids widely dispersed among aerobic spore-forming bacilli. B. subtilis RM125 (168 strain) transformants which carry a varying number of tetBS908 sequences in a tandem array on the chromosome were constructed and examined for their TcR level. A nearly proportional relationship between the TcR level and copy number of tetBS908 existed.

Molecular cloning of a tetracycline-resistance determinant from Bacillus subtilis chromosomal DNA and its expression in Escherichia coli and B. subtilis

Bacillus subtilis GSY908 DNA fragments (5.1 and 4.4 kilobase pairs (kb)) containing a tetracycline-resistance determinant were cloned in Escherichia coli using a shuttle plasmid vector pLS353. Restriction endonucelase analysis showed that the 4.4 kb fragment is a spontaneous deletion derivative of the 5.1 kb fragment. E. coli tetracycline-resistance transformants carrying pLS353 with the 5.1 kb fragment (named pTBS1) and that with 4.4 kb fragment (pTBS1.1) could grow at tetracycline concentrations up to 80 and 50 micrograms per ml, respectively. B. subtilis MI112 and RM125 were transformed by pTBS1, resulting in isolation of transformants of MI112 maintaining pTBS1 and RM125 maintaining either pTBS1 or pTBS1.1. Maximum tetracycline concentrations permitting growth of plasmidless MI112 and MI112 with pTBS1 were 4 and 10 micrograms per ml, respectively, while those of plasmidless RM125, RM125 with pTBS1 and RM125 with pTBS1.1 were 7, 50 and 80 micrograms per ml, respectively. It was interesting to note that the tetracycline-resistance level in E. coli conferred by the 5.1 kb fragment is higher than that conferred by the 4.4 kb fragment, but in B. subtilis the 4.4 kb fragment, in contrast, confers a higher level of tetracycline resistance. The level of tetracycline resistance in B. subtilis conferred by the cloned determinant clearly depends on the host strain. The tetracycline resistance conferred by the cloned determinant was associated with decreased accumulation of the drug into the cells. However, it was constitutive in E. coli, but inducible in B. subtilis. The cloned tetracycline-resistance determinant was detected specifically on the chromosome of B. subtilis Marburg 168 derivatives.

Intermolecular recE4-independent recombination in Bacillus subtilis: formation of plasmid pKBT1

The plasmid pKBT1 was derived by in vivo recE4-independent recombinational event(s) yielding a structure containing regions of plasmid and chromosomal origin. BamHI digests of plasmid pUB110 (Kanr/Neor) and Bg/II digests of pTL12 (Tmpr, leuA) were mixed, ligated and used to transform competent cells of a recE4 strain of Bacillus subtilis. Kanamycin-resistant transformants were electrophoretically screened for hybrid plasmids. Plasmid pKBT1 (8.0 kb) was smaller than pTL12 (10.4 kb) but larger than monomeric pUB110 (4.5 kb). Plasmid pKBT1 was stably maintained in recE4 strains of B. subtilis and conferred kanamycin resistance but did not specify trimethoprim resistance or leucine prototrophy. At least 86% of the pUB110 monomer length was present in pKBT1 and was completely contained within a single 5.58 kb HindIII fragment. The other segment of pKBT1 was of chromosomal origin as evidenced by lack of homology to pTL12 and strong hybridization to B. subtilis chromosomal DNA. At least one of the in vivo recE4-independent event(s) which produced pKBT1 must have involved intermolecular recombination between transforming and chromosomal DNA. This finding differs from previous reports in which recE4-independent recombination involving pUB110 sequences was a strictly intramolecular event.

Sequence and properties of pIM13, a macrolide-lincosamide-streptogramin B resistance plasmid from Bacillus subtilis

We initiated a study of pIM13, a multicopy, macrolide-lincosamide-streptogramin B (MLS) plasmid first isolated from a strain of Bacillus subtilis and described by Mahler and Halvorson (J. Gen. Microbiol. 120:259-263, 1980). The copy number of this plasmid was about 200 in B. subtilis and 30 in Staphylococcus aureus. The MLS resistance determinant of pIM13 was shown to be highly homologous to ermC, an inducible element on the S. aureus plasmid pE194. The product of the pIM13 determinant was similar in size to that of ermC and immunologically cross-reactive with it. The MLS resistance of pIM13 was expressed constitutively. The complete base sequence of pIM13 is presented. The plasmid consisted of 2,246 base pairs and contained two open reading frames that specified products identified in minicell extracts. One was a protein of 16,000 molecular weight, possibly required for replication. The second was the 29,000-molecular-weight MLS resistance methylase. The regulatory region responsible for ermC inducibility was missing from pIM13, explaining its constitutivity. The remainder of the pIM13 MLS determinant was nearly identical to ermC. The ends of the region of homology between pIM13 and pE194 were associated with hyphenated dyad symmetries. A segment partially homologous to one of these termini on pIM13 and also associated with a dyad was found in pUB110 near the end of a region of homology between that plasmid and pBC16. The entire sequence of pIM13 was highly homologous to that of pE5, an inducible MLS resistance plasmid from S. aureus that differs from pIM13 in copy control.