Plasmid-Chromosome Crosstalk in Staphylococcus aureus: A Horizontally Acquired Transcription Regulator Controls Polysaccharide Intercellular Adhesin-Mediated Biofilm Formation.

Gabriella Marincola,Freya D.R Wencker,Ann-Katrin Kieninger,Andrea T Feßler,Greta Jaschkowitz,Stefan Schwarz,Wilma Ziebuhr

doi:10.3389/fcimb.2021.660702

Abstract

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) of clonal complex CC398 typically carry various antimicrobial resistance genes, many of them located on plasmids. In the bovine LA-MRSA isolate Rd11, we previously identified plasmid pAFS11 in which resistance genes are co-localized with a novel ica-like gene cluster, harboring genes required for polysaccharide intercellular adhesin (PIA)-mediated biofilm formation. The ica genes on pAFS11 were acquired in addition to a pre-existing ica locus on the S. aureus Rd11 chromosomal DNA. Both loci consist of an icaADBC operon and icaR, encoding a corresponding icaADBC repressor. Despite carrying two biofilm gene copies, strain Rd11 did not produce PIA and transformation of pAFS11 into another S. aureus strain even slightly diminished PIA-mediated biofilm formation. By focusing on the molecular background of the biofilm-negative phenotype of pAFS11-carrying S. aureus, we identified the pAFS11-borne ica locus copy as functionally fully active. However, transcription of both plasmid- and core genome-derived icaADBC operons were efficiently suppressed involving IcaR. Surprisingly, although being different on the amino acid sequence level, the two IcaR repressor proteins are mutually replaceable and are able to interact with the icaA promoter region of the other copy. We speculate that this regulatory crosstalk causes the biofilm-negative phenotype in S. aureus Rd11. The data shed light on an unexpected regulatory interplay between pre-existing and newly acquired DNA traits in S. aureus. This also raises interesting general questions regarding functional consequences of gene transfer events and their putative implications for the adaptation and evolution of bacterial pathogens.

Highlights

Staphylococcus aureus is a common human and animal pathogen, causing a wide range of clinical manifestations (Tong et al, 2015; Ballhausen et al, 2017)
The two ica loci from S. epidermidis are highly divergent from ica operon from pAFS11 (icapAF), while they are closer related to ica from S. aureus
The data suggest that the two ica loci present in Rd11 are of different genetic origin, with icapAF most likely being derived from another species for which S. sciuri (M. sciuri) is a putative candidate

Summary

Introduction

Staphylococcus aureus is a common human and animal pathogen, causing a wide range of clinical manifestations (Tong et al, 2015; Ballhausen et al, 2017). Ica locus homologs exist in phylogenetically unrelated bacteria such as Escherichia coli (Wang et al, 2004), suggesting an eminent role of the factor in the evolution of bacterial biofilm functions In these organisms, PIA is often referred to as PNAG (poly-1,6-Nacetylglucosamine). S. aureus Rd11 is a livestock-associated (LA)-MRSA strain of sequence type ST398, a clonal lineage known for its potential to carry a broad range of both common and novel antibiotic resistance genes, many of which located on plasmids (Kadlec et al, 2012; Feßler et al, 2018). We identified an unexpected IcaR-mediated regulatory crosstalk between the plasmid-borne and chromosomally encoded ica loci, resulting in downregulation of biofilm formation We discuss these findings in the context of co-evolution of virulence and resistance traits and raise the question of how genes newly acquired by HGT might become integrated into the regulatory network of host bacteria

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