Gram-stain-positive Bacteria

Question 1

Staphylococcus aureus
What is methicillin-resistant Staphylococcus aureus (MRSA)?
What is vancomycin-resistant Staphylococcus aureus (VRSA)?
Where is S. aureus present?

Answer 1

Staphylococcus, balls that look like bunches of grapes
* Gram-stain-positive cocci, form random grape-like clusters.
* found on skins of most Mammalia – forehead, groin and nasal mucosa in Homo sapiens L.
* pathovars (strains) include methicillin-resistant Staphylococcus aureus (MRSA)= which has strong resistance to β-lactam antimicrobials inc. penicillins, methicillin etc

*vancomycin-resistant Staphylococcus aureus (VRSA)= It resists glycopeptide antimicrobials inc. vancomycin.

• S. aureus is present in folliculitis (boils, pimples etc.), impetigo etc. or more serious infections like osteomyelitis.
• causes ‘bumblefoot’ in Gallus gallus subsp. domesticus L. (chickens)
• easily transferred from mouth/nose of Canis lupus.

Question 2

Streptococcus pyogenes

Answer 2

• Gram-stain-positive cocci, form chains.
• up to 5 % of healthy H. sapiens L. have Spc. pyogenes in rectum, throat or vagina. Part of normal skin flora.
• many hemolytic strains – can lyse red blood cells.
• causative agent of “strep throat” – often happens just after influenza (in adults) or chickenpox (in children).
• can cause serious conditions too e.g. pneumonia, necrotising fasciitis and maternal Streptococcus pyogenes infection in late pregnancy.
• other species in same genus associated with horses (Streptococcus equinus), dogs (Streptococcus canis), pigs (Streptococcus porci).

Question 3

Lactobacillus acidophilus

Answer 3

• Gram-stain-positive rods. Grow below pH 5.
• abundant in oral cavity of Homo sapiens L. and in upper GI tract.
• microaerophilic.
• ferments sugars by homolactic fermentation – pyruvate from glycolysis is oxidised to lactic acid – no CO2/H2 produced.
• used in commercial dairy industry to ferment sugars in milk, producing yoghurts, cheeses etc. Also key in production of sourdough breads.
• used as a probiotic to treat irritable bowel syndrome and to out-compete Helicobacter pylori.
• used in experimental probiotics for the neovagina to give a more vagina-like microflora.

Question 4

Lactobacillus vaginalis

Answer 4

• Gram-stain-positive rods. Grow below pH 5.
• occurs in of H. sapiens L. vaginae – albeit in fewer than 1 % of
  people with vaginae – in spite of name, many other Lactobacillus spp. are the more dominant vaginal flora.
• microaerophilic.
• ferments sugars by heterolactic fermentation – pyruvate from glycolysis is oxidised to lactic acid, H2 and CO2 produced.
• used in lactobacillus vaccination in parts of Europe to deal with persistent infections of the vagina.

Question 5

Mycobacterium tuberculosis

Answer 5

• Gram-stain-positive rods. Very large cells – acid-fast cell-walls – mycolic acids are present.
• obligate aerobe and requires high pO2, thus colonises the lung tissue of Mammalia.
• is taken up into phagosomes of macrophages but resists attack, eventually resulting in formation of granuloma in the lungs – “tuberculosis” (TB).
• TB of the spine (Pott disease) and brain (Rich foci) are also known.
• similar species found in domestic cattle (M. bovis), seals.
• treated with triple therapy of antimicrobials for up to a year.
• BCG based on M. bovis (from cattle) worked well in some countries but in others, it has increased risk of infection.
• major cause of death in end-stage AIDS and infection is more likely in HIV positive than HIV-negative people

Question 6

Deinococcus radiodurans
What is different about its cell wall?

Answer 6

resisting radiation.
* Gram-stain-positive cocci found in tetrads.
* isolated from a can of baked beans in the 1950s.
* Obligate aerobe
* resists up to 5,000 Gy at a time (> 30 Gy will kill a human in under 48 h. It can resist α, β, γ/X-radiation, UV radiation and neutron beams.
* also resists desiccation (drying out)
* Stains Gram-stain-positive but the cell wall is more like a modified Gram-stain-negative cell wall.
* oxidises Mn2+ outside of the cell into MnO2 which coats the cell wall – Mn atoms act as radiation shielding.
* genome is spread over 2 chromosomes, a chromid and a plasmid – cells usually have 4 complete copies of all of this at any time – up to 10 copies when rapidly dividing.
Chromosomes are supercoiled into tight toroids to give structural rigidity.

(compare with Thermococcus gammatolerans).

Question 7

What species was used to generate the Mycobacterium tuberculosis vaccine and what its host organism?

Answer 7

Mycobacterium bovis, cattle