Artificial elimination of drug resistance from group A beta-hemolytic streptococci

Strategies to combat antimicrobial resistance: anti-plasmid and plasmid curing

Antimicrobial resistance (AMR) is a global problem hindering treatment of bacterial infections, rendering many aspects of modern medicine less effective. AMR genes (ARGs) are frequently located on plasmids, which are self-replicating elements of DNA. They are often transmissible between bacteria, and some have spread globally. Novel strategies to combat AMR are needed, and plasmid curing and anti-plasmid approaches could reduce ARG prevalence, and sensitise bacteria to antibiotics. We discuss the use of curing agents as laboratory tools including chemicals (e.g. detergents and intercalating agents), drugs used in medicine including ascorbic acid, psychotropic drugs (e.g. chlorpromazine), antibiotics (e.g. aminocoumarins, quinolones and rifampicin) and plant-derived compounds. Novel strategies are examined; these include conjugation inhibitors (e.g. TraE inhibitors, linoleic, oleic, 2-hexadecynoic and tanzawaic acids), systems designed around plasmid incompatibility, phages and CRISPR/Cas-based approaches. Currently, there is a general lack of in vivo curing options. This review highlights this important shortfall, which if filled could provide a promising mechanism to reduce ARG prevalence in humans and animals. Plasmid curing mechanisms which are not suitable for in vivo use could still prove important for reducing the global burden of AMR, as high levels of ARGs exist in the environment.

Characterization of a small plasmid determining resistance to erythromycin, lincomycin, and vernamycin Balpha in a strain of Streptococcus sanguis isolated from dental plaque

A plasmid determining resistance to erythromycin (inducible), lincomycin, and vernamycin B(alpha) was isolated from a strain of Streptococcus sanguis isolated from dental plaque of a patient who had been on erythromycin therapy for an extended period of time. The plasmid, designated pAM77, had a molecular weight of 4.5 x 10(6) and was present to the extent of one to two copies per bacterial chromosome. The plasmid could transform S. sanguis strain Challis.

R plasmids in Streptococcus agalactiae (group B)

Two plasmids determining resistance to tetracycline (RIP500) and to chloramphenicol, erythromycin, lincomycin, and pristinamycin I (RIP501) were isolated from a strain of Streptococcus agalactiae. The frequency-of-resistance loss is very low for RIP500 (<3 x 10(4)) but higher for RIP501 (the efficiency was dependent upon the curing agents and incubation temperature and varied between 0.5 and 96%). Derivatives susceptible to all drugs were also obtained. RIP500 and RIP501 have similar molecular weights (17.9 x 10(6) and 20 x 10(6), respectively) and represent different percentages of total deoxyribonucleic acid (0.4 and 4%, respectively). The number of copies of RIP500 and RIP501 per cell is different, and these plasmids are likely replicated under different kinds of control (stringent and/or relaxed). No plasmid deoxyribonucleic acid was found in a derivative of strain B96 susceptible to all drugs.

Transformation of Streptococcus sanguis Challis by plasmid deoxyribonucleic acid from Streptococcus faecalis

Plasmid deoxyribonucleic acid (DNA) from Streptococcus faecalis, strain DS5, was transferred to the Challis strain of Streptococcus sanguis by transformation. Two antibiotic resistance markers carried by the beta plasmid from strain DS5, erythromycin and lincomycin, were transferred to S. sanguis at a maximum frequency of 1.8 x 10-5/colony-forming unit. Approximately 70% of the covalently closed circular DNA isolated from transformant cultures by dye buoyant density gradients was shown to be hybridizable to beta plasmid DNA. Two major differences were observed between the beta plasmid from S. faecalis and the plasmid isolated from transformed S. sanguis: (i) the beta plasmid from strain DS5 sedimented in velocity gradients at 43S, whereas the covalently closed circular DNA from transformed Challis sedimented at 41S, suggesting a 1.5-Mdal deletion from the beta plasmid occurred; (ii) although the 43S beta plasmid remained in the supercoiled configuration for several weeks after isolation, the 41S plasmid was rapidly converted to a linear double-stranded molecule. Attempts to transform S. sanguis with the alpha plasmid from S. faecalis, strain DS5, were unsuccessful.

Genetic basis of streptococcin A-FF22 production

Spontaneous, low-frequency loss of ability to produce streptococcin A-FF22 (SA) by group A streptococcus strain FF22 was observed. The proportion of non-SA-producing (SA(-)) derivatives occurring in strain FF22 cultures grown in Todd Hewitt broth supplemented with 1% of yeast extract (THBY) was increased on treatment with ethidium bromide, acriflavin, or rifampin. The highest incidence of SA(-) organisms, however, was found in untreated THBY cultures that had been aging by incubation at 37 degrees C for several months. The possibility of selective effects in these experiments, operating to enhance the apparent frequency of SA(-) bacteria, was discounted. The survival of SA(-) derivatives in association with populations of SA(+) bacteria was dependent upon the use of culture conditions inimical to SA activity, since a consistent finding was that the loss of ability to produce SA was associated with loss of immunity to the killing action of this bacteriocin. Whereas selective killing of SA(-) derivatives was evident in mixed cultures of SA(+) and SA(-) strains in tryptic soy broth, no such effect was demonstrable in THBY. In these experiments, elimination of SA(-) cells seemed directly related to the presence of active SA. Purified clones of SA(-) substrains did not seem revertible to SA production, either spontaneously or on treatment with nitrosoguanidine. It is suggested that the property of production of SA by group A streptococcus strain FF22, together with that of host cell immunity to the homologous bacteriocin, may be mediated by plasmid-borne genetic determinants.