🦠 concepts in microbiology: bacteria

Question 1

What is Staphylococcus aureus?

Answer 1

Gram positive bacteria characterised by grape-like gram stain appearance.

Question 2

What is blood agar?

Answer 2

A red coloured non-selective agar which grows all types of bacteria, making it difficult to pick up pathogens of interest.

Question 3

What is mannitol salt agar?

Answer 3

A commonly used type of selective agar which grows a specific type of bacteria.

High [salt] inhibits bacterial growth; S. aureus ferments mannitol (sugar) → pH lowered → acidic → pH indicator in agar turns yellow.

Question 4

What does aerobic mean?

Answer 4

Requires oxygen.

Question 5

What does capnophilic mean?

Answer 5

Requires carbon dioxide.

Question 6

What does facultative mean?

Answer 6

Can live and grow in the presence or absence of oxygen.

Example: S. aureus.

Question 7

What does microaerophilic mean?

Answer 7

Requires small amounts of oxygen.

Question 8

What does anaerobic mean?

Answer 8

Requires an absence of oxygen; O2 is toxic to some of them.

Question 9

What is the gram stain reaction?

Answer 9

A laboratory test that checks for bacteria at the site of a suspected infection or in certain bodily fluids and classifies them as either gram + or -.

Steps: 1. Apply primary stain (crystal violet); 2. Add mordant (Lugol’s iodine); 3. Rapid decolourization with acetone; 4. Counterstain with safranin (red dye).

Question 10

What characterizes gram positive bacteria cell envelope?

Answer 10

Has a thick layer of peptidoglycan but lacks outer membrane.

Question 11

What characterizes gram negative bacteria cell envelope?

Answer 11

Has a thin layer of peptidoglycan but is surrounded by an outer membrane containing lipopolysaccharide.

Question 12

What is transpeptidase?

Answer 12

An enzyme that catalyses the cross-linking of peptidoglycan in bacterial cell walls; a penicillin binding protein.

Targets for penicillin-based antibiotics: cross linking stopped → bacteria less resistant to osmotic pressure → increased chance of cell wall lysis.

Question 13

What is peptidoglycan?

Answer 13

A polymer that forms the cell wall of most bacteria, providing structural support and protection.

Thick on gram + cells, thin on gram - cells.

Question 14

What are exotoxins?

Answer 14

Toxins released by gram +ve and gram -ve bacteria (mostly +ve) which destroys cells or disrupts normal cell metabolism.

Example: panton-valentine leukicidin (PVL) secreted by S. aureus.

Question 15

What are endotoxins?

Answer 15

Toxins (lipopolysaccharides) found in the cell wall of Gram-negative bacteria which induces inflammation & fever in patients.

Example: Prevotella intermedia, E. coli systemic infection.

Question 16

What is panton-valentine leukicidin (PVL)?

Answer 16

An exotoxin produced by certain types of Staphylococcus aureus which can kill white blood cells and cause damage to skin and deeper tissues.

1. Two components of PVL secreted from S. aureus; 2. Assemble into pore-forming heptamer (7) on PMN leukocyte membranes; 3. High PVL concentrations cause PMN lysis; 4. Lysed PMNs → inflammatory response → tissue necrosis.

Question 17

What is E. Coli systemic infection?

Answer 17

The systemic reaction to endotoxin (cytokines) or lipopolysaccharides associated with urinary tract source which can lead to disseminated intravascular coagulation and death.

Question 18

What does cocci shaped mean?

Answer 18

Round shaped bacteria.

Can be clumped or streptococci (chains).

Question 19

What does bacilli shaped mean?

Answer 19

Rod/rice shaped bacteria.

Question 20

What does spiral shaped mean?

Answer 20

Shape of bacteria with very thin cell walls, difficult to spot.

Example: acute ulcerative gingivitis (very painful).

Question 21

What are mycobacteria?

Answer 21

Type of bacteria which is an exception to the gram stain (resistant to decolorisation by the acid-alcohol solutions used in Gram staining), can be in contaminated tattoo ink, environmental water, soil & dust.

Question 22

What is Streptococcus mutans?

Answer 22

Example of gram +ve cocci bacteria which are the causative agent of dental caries.

Question 23

What is Neisseria meningitidis?

Answer 23

Example of gram -ve diplococci (appear in 2’s) bacteria which can live in the back of the nose and throat.

Question 24

What is Clostridium tetani?

Answer 24

Example of gram +ve bacilli bacteria which causes extremely painful spinal contractions.

Question 25

What is Prevotella intermedia?

Answer 25

Example of gram -ve bacilli bacteria which are involved in periodontal infections, including gingivitis and periodontitis.

Question 26

What is M. abscessus?

Answer 26

Example of a multidrug-resistant, nontuberculous mycobacteria that is a common soil and water contaminant.