Ian R. Monk

Transforming the Untransformable: Application of Direct Transformation To Manipulate Genetically Staphylococcus aureus and Staphylococcus epidermidis

ABSTRACT The strong restriction barrier present in Staphylococcus aureus and Staphylococcus epidermidis has limited functional genomic analysis to a small subset of strains that are amenable to genetic manipulation. Recently, a conserved type IV restriction system termed SauUSI (which specifically recognizes cytosine methylated DNA) was identified as the major barrier to transformation with foreign DNA. Here we have independently corroborated these findings in a widely used laboratory strain of S. aureus. Additionally, we have constructed a DNA cytosine methyltransferase mutant in the high-efficiency Escherichia coli cloning strain DH10B (called DC10B). Plasmids isolated from DC10B can be directly transformed into clinical isolates of S. aureus and S. epidermidis. We also show that the loss of restriction (both type I and IV) in an S. aureus USA300 strain does not have an impact on virulence. Circumventing the SauUSI restriction barrier, combined with an improved deletion and transformation protocol, has allowed the genetic manipulation of previously untransformable strains of these important opportunistic pathogens. IMPORTANCE Staphylococcal infections place a huge burden on the health care sector due both to their severity and also to the economic impact of treating the infections because of prolonged hospitalization. To improve the understanding of Staphylococcus aureus and Staphylococcus epidermidis infections, we have developed a series of improved techniques that allow the genetic manipulation of strains that were previously refractory to transformation. These developments will speed up the process of mutant construction and increase our understanding of these species as a whole, rather than just a small subset of strains that could previously be manipulated. Staphylococcal infections place a huge burden on the health care sector due both to their severity and also to the economic impact of treating the infections because of prolonged hospitalization. To improve the understanding of Staphylococcus aureus and Staphylococcus epidermidis infections, we have developed a series of improved techniques that allow the genetic manipulation of strains that were previously refractory to transformation. These developments will speed up the process of mutant construction and increase our understanding of these species as a whole, rather than just a small subset of strains that could previously be manipulated.

Tools for Functional Postgenomic Analysis of Listeria monocytogenes

ABSTRACT We describe the development of genetic tools for regulated gene expression, the introduction of chromosomal mutations, and improved plasmid transfer by electroporation in the food-borne pathogen Listeria monocytogenes. pIMK, a kanamycin-resistant, site-specific, integrative listeriophage vector was constructed and then modified for overexpression (pIMK2) or for isopropyl-β-d-thiogalactopyranoside (IPTG)-regulated expression (pIMK3 and pIMK4). The dynamic range of promoters was assessed by determining luciferase activity, P60 secretion, and internalin A-mediated invasion. These analyses demonstrated that pIMK4 and pIMK3 have a stringently controlled dynamic range of 540-fold. Stable gene overexpression was achieved with pIMK2, giving a range of expression for the three vectors of 1,350-fold. The lactococcal pORI280 system was optimized for the generation of chromosomal mutations and used to create five new prfA star mutants. The combination of pIMK4 and pORI280 allowed streamlined creation of “IPTG-dependent” mutants. This was exemplified by creation of a clean deletion mutant with deletion of the universally essential secA gene, and this mutant exhibited a rapid loss of viability upon withdrawal of IPTG. We also improved plasmid transfer by electroporation into three commonly used laboratory strains of L. monocytogenes. A 125-fold increase in transformation efficiency for EGDe compared with the widely used protocol of Park and Stewart (S. F. Park and G. S. Stewart, Gene 94:129-132, 1990) was observed. Maximal transformation efficiencies of 5.7 × 106 and 6.7 × 106 CFU per μg were achieved for EGDe and 10403S, respectively, with a replicating plasmid. An efficiency of 2 × 107 CFU per μg is the highest efficiency reported thus far for L. monocytogenes F2365.

AgrD‐dependent quorum sensing affects biofilm formation, invasion, virulence and global gene expression profiles in Listeria monocytogenes

The Listeria monocytogenes Agr peptide‐sensing system has been analysed by creating a deletion mutant in agrD, the structural gene for the putative quorum‐sensing peptide. The ΔagrD mutant displayed significantly reduced biofilm formation, a defect which could be restored by genetic or physical complementation. A reduced invasion of Caco‐2 intestinal epithelial cells was observed for the ΔagrD mutant while phagocytosis by THP‐1 macrophages was unaffected. Additionally, the level of internalin A (InlA) in the cell wall was decreased in the ΔagrD mutant. Expression profiling of virulence genes (hlyA, actA, plcA, prfA and inlA) identified a finely tuned regulation which resulted in an impaired virulence response in the ΔagrD mutant. The mutant is also significantly attenuated for virulence in mice, as revealed by bioluminescent in vivo imaging. On day 3 post infection, systemic dissemination to livers and spleens had occurred for the wild type, whereas the ΔagrD mutant remained localized to the liver. Microarray analysis identified 126 and 670 genes as significantly regulated in exponential and stationary phase respectively. The results presented here suggest that peptide sensing plays an important role in the biology of L. monocytogenes, with relevant phenotypes in both the saprophytic and parasitic lifecycles.

Improved Luciferase Tagging System for Listeria monocytogenes Allows Real-Time Monitoring In Vivo and In Vitro

ABSTRACT An improved system for luciferase tagging Listeria monocytogenes was developed by constructing a highly active, constitutive promoter. This construct gave 100-fold-higher activity in broth than any native promoter tested and allowed for imaging of lux-tagged L. monocytogenes in food products, during murine infections, and in tumor targeting studies.

Nasal colonisation by Staphylococcus aureus depends upon clumping factor B binding to the squamous epithelial cell envelope protein loricrin.

Staphylococcus aureus asymptomatically colonises the anterior nares, but the host and bacterial factors that facilitate colonisation remain incompletely understood. The S. aureus surface protein ClfB has been shown to mediate adherence to squamous epithelial cells in vitro and to promote nasal colonisation in both mice and humans. Here, we demonstrate that the squamous epithelial cell envelope protein loricrin represents the major target ligand for ClfB during S. aureus nasal colonisation. In vitro adherence assays indicated that bacteria expressing ClfB bound loricrin most likely by the “dock, lock and latch” mechanism. Using surface plasmon resonance we showed that ClfB bound cytokeratin 10 (K10), a structural protein of squamous epithelial cells, and loricrin with similar affinities that were in the low µM range. Loricrin is composed of three separate regions comprising GS-rich omega loops. Each loop was expressed separately and found to bind ClfB, However region 2 bound with highest affinity. To investigate if the specific interaction between ClfB and loricrin was sufficient to facilitate S. aureus nasal colonisation, we compared the ability of ClfB+ S. aureus to colonise the nares of wild-type and loricrin-deficient (Lor−/−) mice. In the absence of loricrin, S. aureus nasal colonisation was significantly impaired. Furthermore a ClfB− mutant colonised wild-type mice less efficiently than the parental ClfB+ strain whereas a similar lower level of colonisation was observed with both the parental strain and the ClfB− mutant in the Lor−/− mice. The ability of ClfB to support nasal colonisation by binding loricrin in vivo was confirmed by the ability of Lactococcus lactis expressing ClfB to be retained in the nares of WT mice but not in the Lor−/− mice. By combining in vitro biochemical analysis with animal model studies we have identified the squamous epithelial cell envelope protein loricrin as the target ligand for ClfB during nasal colonisation by S. aureus.

Novel Luciferase Reporter System for In Vitro and Organ-Specific Monitoring of Differential Gene Expression in Listeria monocytogenes

ABSTRACT In this paper we describe construction of a luciferase-based vector, pPL2lux, and use of this vector to study gene expression in Listeria monocytogenes. pPL2lux is a derivative of the listerial integration vector pPL2 and harbors a synthetic luxABCDE operon encoding a fatty acid reductase complex (LuxCDE) involved in synthesis of the fatty aldehyde substrate for the bioluminescence reaction catalyzed by the LuxAB luciferase. We constructed pPL2lux derivatives in which the secA and hlyA promoters were translationally fused to luxABCDE and integrated as a single copy into the chromosome of L. monocytogenes EGD-e. Growth experiments revealed that hlyA was expressed predominantly in the stationary phase in LB medium buffered at pH 7.4, whereas secA expression could be detected in the exponential growth phase. Moreover, the correlation between luciferase activity and transcription levels, as determined by reverse transcriptase PCR, was confirmed using conditions known to lead to repression and activation of hemolysin expression (addition of cellobiose and activated charcoal, respectively). Furthermore, hemolysin expression could be monitored in real time during invasion of an intact monolayer of C2Bbe1 (Caco-2-derived) cells. Finally, hemolysin expression could be detected in the livers, spleens, and kidneys of mice 3 days postinfection. These experiments clearly established the effectiveness of pPL2lux as a quantitative reporter system for real-time, noninvasive evaluation of gene expression in L. monocytogenes.

Inactivation of Listeria monocytogenes/Flavobacterium spp. biofilms using chlorine: impact of substrate, pH, time and concentration

Aims: To determine the effect of chlorine on mixed bacterial biofilms on stainless steel (SS) and conveyor belt surfaces. 
Methods and Results: Biofilms were exposed to pH‐adjusted (6·5) and non‐pH‐adjusted solutions of chlorine (200, 400 and 600 ppm) for either 2, 10 or 20 min and survivors enumerated. There were significant differences in cell death relating to chlorine concentration and exposure time for the cells attached to the SS, with solutions adjusted to pH 6·5 being more effective at reducing numbers. In contrast, on conveyor belt surfaces cell numbers decreased by less than two logs after 20 min regardless of treatment. 
Conclusions: Chlorine effectiveness is dependent on its concentration, solution pH, exposure time, the nature of the surface and the microbial species present. 
Significance and Impact of Study: In the interests of food safety it is important that sanitizer users are aware of the conditions that effect their performance.

Morphotypic Conversion in Listeria monocytogenes Biofilm Formation: Biological Significance of Rough Colony Isolates

ABSTRACT Adherence to a stainless steel surface selected isolates of Listeria monocytogenes with enhanced surface colonization abilities and a change in phenotype from the common smooth colony morphology to a succession of rough colony morphotypes. Growth in broth culture of the best-adapted, surface-colonizing rough colony morphotype gave a smooth colony revertant. Comparative analysis revealed that the smooth and rough variants had similar phenotypic and biochemical characteristics (e.g., identical growth rates and tolerances to antibiotics and environmental stressors). Rough colony isolates, however, failed to coordinate motility or induce autolysis. The defect in autolysis of rough colony isolates, which involved impaired cellular localization of several peptidoglycan-degrading enzymes, including cell wall hydrolase A (CwhA), suggested a link to a secretory pathway defect. The genetic basis for the impairment was studied at the level of the accessory secretory pathway component SecA2. DNA sequencing of the secA2 gene in smooth and rough colony isolates found no mutations in the coding or promoter regions. Analysis of SecA2 expression with an integrated secA2-FLAG tag construct found the protein to be upregulated in the rough and revertant backgrounds compared to the parental smooth colony isolate. A compensatory mechanism involving the SecA2 secretion pathway components is postulated to control smooth to rough interconversion of L. monocytogenes. Such phenotypic variation may enhance the ability of this opportunistic pathogen to colonize environments as diverse as processing surfaces, food products, and animal hosts.

A novel Listeria monocytogenes-based DNA delivery system for cancer gene therapy.

Bacteria-mediated transfer of plasmid DNA to mammalian cells (bactofection) has been shown to have significant potential as an approach to express heterologous proteins in various cell types. This is achieved through entry of the entire bacterium into cells, followed by release of plasmid DNA. In a murine model, we show that Listeria monocytogenes can invade and spread in tumors, and establish the use of Listeria to deliver genes to tumors in vivo. A novel approach to vector lysis and release of plasmid DNA through antibiotic administration was developed. Ampicillin administration facilitated both plasmid transfer and safety control of vector. To further improve on the gene delivery system, we selected a Listeria monocytogenes derivative that is more sensitive to ampicillin, and less pathogenic than the wild-type strain. Incorporation of a eukaryotic-transcribed lysin cassette in the plasmid further increased bacterial lysis. Successful gene delivery of firefly luciferase to growing tumors in murine models and to patient breast tumor samples ex vivo was achieved. The model described encompasses a three-phase treatment regimen, involving (1) intratumoral administration of vector followed by a period of vector spread, (2) systemic ampicillin administration to induce vector lysis and plasmid transfer, and (3) systemic administration of combined moxifloxacin and ampicillin to eliminate systemic vector. For the first time, our results reveal the potential of Listeria monocytogenes for in vivo gene delivery.

Complete Bypass of Restriction Systems for Major Staphylococcus aureus Lineages

ABSTRACT Staphylococcus aureus is a prominent global nosocomial and community-acquired bacterial pathogen. A strong restriction barrier presents a major hurdle for the introduction of recombinant DNA into clinical isolates of S. aureus. Here, we describe the construction and characterization of the IMXXB series of Escherichia coli strains that mimic the type I adenine methylation profiles of S. aureus clonal complexes 1, 8, 30, and ST93. The IMXXB strains enable direct, high-efficiency transformation and streamlined genetic manipulation of major S. aureus lineages. IMPORTANCE The genetic manipulation of clinical S. aureus isolates has been hampered due to the presence of restriction modification barriers that detect and subsequently degrade inappropriately methylated DNA. Current methods allow the introduction of plasmid DNA into a limited subset of S. aureus strains at high efficiency after passage of plasmid DNA through the restriction-negative, modification-proficient strain RN4220. Here, we have constructed and validated a suite of E. coli strains that mimic the adenine methylation profiles of different clonal complexes and show high-efficiency plasmid DNA transfer. The ability to bypass RN4220 will reduce the cost and time involved for plasmid transfer into S. aureus. The IMXXB series of E. coli strains should expedite the process of mutant construction in diverse genetic backgrounds and allow the application of new techniques to the genetic manipulation of S. aureus. The genetic manipulation of clinical S. aureus isolates has been hampered due to the presence of restriction modification barriers that detect and subsequently degrade inappropriately methylated DNA. Current methods allow the introduction of plasmid DNA into a limited subset of S. aureus strains at high efficiency after passage of plasmid DNA through the restriction-negative, modification-proficient strain RN4220. Here, we have constructed and validated a suite of E. coli strains that mimic the adenine methylation profiles of different clonal complexes and show high-efficiency plasmid DNA transfer. The ability to bypass RN4220 will reduce the cost and time involved for plasmid transfer into S. aureus. The IMXXB series of E. coli strains should expedite the process of mutant construction in diverse genetic backgrounds and allow the application of new techniques to the genetic manipulation of S. aureus.