Conjugative IncC Plasmid Entry Triggers the SOS Response and Promotes Effective Transfer of the Integrative Antibiotic Resistance Element SGI1

Two birds with one stone: SGI1 can stabilize itself and expel the IncC helper by hijacking the plasmid parABS system

The SGI1 family integrative mobilizable elements, which are efficient agents in distribution of multidrug resistance in Gammaproteobacteria, have a complex, parasitic relationship with their IncC conjugative helper plasmids. Besides exploiting the transfer apparatus, SGI1 also hijacks IncC plasmid control mechanisms to time its own excision, replication and expression of self-encoded T4SS components, which provides advantages for SGI1 over its helpers in conjugal transfer and stable maintenance. Furthermore, SGI1 destabilizes its helpers in an unknown, replication-dependent way when they are concomitantly present in the same host. Here we report how SGI1 exploits the helper plasmid partitioning system to displace the plasmid and simultaneously increase its own stability. We show that SGI1 carries two copies of sequences mimicking the parS sites of IncC plasmids. These parS-like elements bind the ParB protein encoded by the plasmid and increase SGI1 stability by utilizing the parABS system of the plasmid for its own partitioning, through which SGI1 also destabilizes the helper plasmid. Furthermore, SGI1 expresses a small protein, Sci, which significantly strengthens this plasmid-destabilizing effect, as well as SGI1 maintenance. The plasmid-induced replication of SGI1 results in an increased copy-number of parS-like sequences and Sci expression leading to strong incompatibility with the helper plasmid.

Resistance in Enteric Shigella and nontyphoidal Salmonella : emerging concepts

PURPOSE OF REVIEW

The emergence of globally resistant enteric Shigella and nontyphoidal Salmonella strains (NTS) has limited the selection of effective drugs, which has become a major challenge for the treatment of infections. The purpose of this review is to provide the current opinion on the antimicrobial-resistant enteric Shigella and nontyphoidal Salmonella .

RECENT FINDINGS

Enteric Shigella and NTS are resistant to almost all classes of antimicrobials in recent years. Those with co-resistance to ciprofloxacin, azithromycin and ceftriaxone, the first-line antibiotics for the treatment of infectious diarrhoea have emerged worldwide. Some of them have caused interregional and international spread by travel, trade, MSM, and polluted water sources. Several strains have even developed resistance to colistin, the last-resort antibiotic used for treatment of multidrug-resistant Gram-negative bacteria infections.

SUMMARY

The drug resistance of enteric Shigella and NTS is largely driven by the use of antibiotics and horizontal gene transfer of mobile genetic elements. These two species show various drug resistance patterns in different regions and serotypes. Hence treatment decisions for Shigella and Salmonella infections need to take into consideration prevalent antimicrobial drug resistance patterns. It is worth noting that the resistance genes such as blaCTX,mph, ermB , qnr and mcr , which can cause resistance to ciprofloxacin, cephalosporin, azithromycin and colistin are widespread because of transmission by IncFII, IncI1, IncI2 and IncB/O/K/Z plasmids. Therefore, continuous global monitoring of resistance in Shigella and Salmonella is imperative.

Effect of the S008-sgaCD operon on IncC plasmid stability in the presence of SGI1-K or absence of an SGI1 variant

An IncC or IncA plasmid is needed to enable transfer of SGI1 type integrative mobilisable elements but an IncC plasmid does not stably co-exist with SGI1. However, the plasmid is stably maintained with SGI1-K, a natural SGI1 deletion variant that lacks the sgaDC genes (S007 and S006) and the upstream open reading frame (S008) found in the SGI1 backbone. Here, the effect of the sgaDC genes and S008 on the stability of an IncC plasmid in an Escherichia coli strain with or without SGI1-K was examined. Co-transcription of the S008 open reading frame with the downstream sgaDC genes was established. When a strain containing SGI1-K complemented with a pK18 plasmid that included S008-sgaDC or sgaDC expressed from the constitutive pUC promoter was grown without antibiotic selection, the resident IncC plasmid was rapidly lost but loss was slower when S008 was present. In contrast, SGI1-K and the S008-sgaDC or sgaDC plasmid were quite stably maintained for >100 generations. However, the high copy number plasmids carrying the SGI1-derived S008-sgaDC or sgaDC genes constitutively expressed could not be introduced into an E. coli strain carrying the IncC plasmid but without SGI1-K. Using equivalent plasmids with S008-sgaDC or sgaDC genes controlled by an arabinose-inducible promoter, under inducing conditions the IncC plasmid was stable but the plasmid containing the SGI1-derived genes was rapidly lost. This unexpected observation indicates that there are multiple interactions between the IncC plasmid and SGI1 in which the transcriptional activator genes sgaDC play a role. These interactions will require further investigation.