Question

Why are some strains of staphylococcus aureus harmless but others can be deadly?

Answer 1

Ans. All Staphylococcus aureus strains exhibit characteristic catalase positive and staphyloxanthium producing nature along with presentation of other intrinsic virulence factors like protein A, teichoic acid, polysaccharide capsule, etc. Note that these virulence factors are encoded by genomic DNA, so are naturally produced by the bacteria. So, all long as the bacteria produces one or more of the virulence factors, it is pathogenic.

# In contrast, the antibiotic-resistance is an outcome of EXTRA-GENOMIC DNA, called PLASMID. So, some strains may or may not have specific antibiotic-resistance gene depending on the presence of absence of such plasmids. Plasmids are not crucial for bacterial survival under normal physiological conditions.

However, again note that the virulence factors are encoded by GENOMIC DNA, so at any time, all strains express most of the virulence factors in them under normal physiological conditions. Due to this, all the strains of S. aureus are pathogenic (better say, opportunistic pathogen) to human due to production of virulence factors from genomic DNA.

# Whether a strain is pathogenic or not ?

Since the virulence factors are encoded by genomic DNA, all the strains are pathogenic to human because all strains possess similar genetic makeup – and – it is quite unlikely that all the genes for virulence factors are mutated (or become non-functional) at the same time.

The pathogenicity of any strain of S. aureus largely depends on the immunity of the host. This bacteria is a part of normal microflora of human skin and nasal cavity, etc. Generally, it does not cause diseases.

However, when the normal microflora is disturbed or the immunity of the host is lowered, the same harmless bacteria becomes pathogenicity and causes disease.