MRSA: Molecular Genetics of Pathogenicity, Virulence and Drug Resistance By Joseph Chidiac Presented to Dr. Sima Tokajian For Advanced Molecular Biology.

Staphylococcus aureus S. aureus is a gram positive bacterium of approximately 1um in diameter, and it is normally found in the human respiratory tract (Crossley & Archer 1997). It can cause a range of illnesses, from minor skin infections to life-threatening diseases such as pneumonia, meningitis, toxic shock syndrome (TSS), bacteremia, sepsis, and others.

Potential Harm It is estimated that 20-30% of the human population are long-term carriers of this germ (Plata et al. 2009). S. aureus is the most infamous cause of Staph infections, which can be widespread and fatal if untreated. It is coagulase positive, meaning it has the ability to clot plasma in blood, and oxidase negative, so they require complex nutrients (Plata et al. 2009). It is also the leading culprit in nosocomial (or hospital acquired) infections.

Cause of Death S. aureus can directly cause Staph infections, usually characterized by abscesses under the skin which require incision and drainage, and sometimes manifest as severe infections of the muscle, heart, joints, bones etc. (Raygada et al. 2009). These abscesses cause very painful tissue necrosis and, in extreme cases, even death.

Methicillin Resistance Since the 1960s, several strains have emerged that possess antibiotic resistance to both the penicillin and Methicillin groups of antibiotics (Feng et al. 2007). Although, drug resistant S. aureus is still susceptible to a combination of Methicillin and lysostaphin, which attacks glycine residues in the peptidoglycan cell wall (Plata et al. 2009). These strains fall into two large categories: CA-MRSA and HA-MRSA (Community or Hospital acquired). CA-MRSA is known to be more virulent than HA-MRSA due to higher expression of virulence genes (Wang et al. 2007).

β-lactamase and PBP2a S. aureus combats the β-lactam group of antibiotics (including Methicillin, oxacillin, nafcillin…) by means of β-lactamase which is encoded by the gene BlaZ (Feng et al. 2007). It counters penicillin and oxacillin with PBP2a (Penicillin Binding Protein) which is encoded by the MecA gene (Feng et al. 2007). Both MecA and BlaZ are found on larger operons containing many other regulatory genes.

β-lactamase and PBP2a MecI inhibits the transcription of mecA and the mecRI-mecA operon. When β-lactam binds the sensor domain of MecRI, the intracellular peptidase cleaves the MecI repressor, which in turn triggers the transcription of mecA and mecI. BlaZ, blaRI, and blaI work in an analogous manner. The BlaI and MecI repressor proteins are interchangeable, but the cleavage of the regulators by BlaRI and MecRI is highly specific (Plata et al. 2009).

SCCmec Different strains of MRSA have different versions of the SCCmec plasmid (or genomic island).

SCCmec Integration The SCCmec plasmid can integrate into the bacterial chromosome by binding the plasmid attachment site (attSCC) to the chromosomal attachment site (attB) to allow for drug resistance genes to be transcribed or for the genes to be replicated in mitotic division (Plata et al. 2009). Chromosomal Casette Recombinase genes (ccrA, ccrB, and ccrC) allow for this integration.

Virulence Determinants Genes coding for virulence factors differ from strain to strain, though there are some general proteins found in most, and certain factors of interest are only found in highly virulent strains. In MRSA strains where additional virulence factors are encoded, this sometimes sacrifices the space otherwise occupied by additional resistance genes (Baba et al. 2002).

(Wang et al. 2007)

(Feng et al. 2007)

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Transposons and Adaptability Transposons, along with insertion sequences, tend to insert themselves into the chromosome by illegitimate recombination, causing a shuffling of the genome and enhanced adaptability to adverse environments for S. aureus (Baba et al. 2002). Strains differ in the number and type of transposons and genomic islands they comprise, and this is due to specific environmental needs (Baba et al. 2002). For example, the bacteriocin operon appears to be indispensible to CA-MRSA, which has to compete with a score of other pathogens . HA-MRSA strains, however, contain a higher number of transposons mainly involved in drug resistance, and it appears that this number increases further the longer a strain is able to propagate in the hospital (Baba et al. 2002).

Latest MRSA News Researchers at Fudan University (Shanghai, China) and the NIH (MD, USA) have identified a new MRSA gene that appears to enhance colonization and pathogenesis. SasX apparently codes for a surface protein in only 3 out of the 43 predominant Asian MRSA strains. This protein substantially enhances nasal colonization, lung disease, abscess formation, and it promotes mechanisms of immune evasion. It was also found to cause intracellular bacterial aggregation and biofilm formation.