Path (Micro) - Exam 1

Question 1

What does it mean if bacteria is gram negative?

Answer 1

The peptidoglycan cell wall is thinner, which means the outer membrane is present and the bacteria will not be very receptive to antibiotics.

The stain will be pink.

Question 2

What are the main gram positive bacteria?

Answer 2

Staph – diplococcic, clusters
Strep – cocci chains
Entero

Question 3

When would we use the catalase test and what would it tell us?

Answer 3

If bacteria is gram positive and we want to find out which one it is.

POSITIVE: If there are oxygen bubbles when exposed to H202, the bacteria contains catalase and is probably STAPH.

NEGATIVE: Strep or Entero

Question 4

When would we use the Coagulase test and what would it tell us?

Answer 4

We would use this test if the bacteria is staph and we want to know what type (i.e. bacteria is already gram positive and positive catalase test).

Positive: S. aureus (which produces coagulase)
Negative: ‘coagulase negative staph’ (‘CNS’)

Question 5

When would we test Haemolytic Properties and what would it tell us?

Answer 5

We would test to determine if alpha or beta haemolytic.

Grow bacteria on blood agar.

• Transparent zone – complete digestion of RBC’s = Beta-haemolysitic
• Olive green agar – partial gigestion of RBC’s = Alpha- haemolysitic Strep/Entero

Question 6

When would we test Antigen Differences and what would it tell us?

Answer 6

We would test it if we want to know what type of B- haemolysitic strep/entero the bacteria is, i.e. positive gram, negative catalase and Beta haemolytic properties.

Used to determine which Lancefield sterotype it is (i.e. A-H and K-V) using Lancefield antisera.

Question 7

What are the sterile body sites? (no commensals or other bacteria)

Answer 7

• blood stream
• cerebrospinal fluid
• bladder
• peritoneal space
• joint space etc

Question 8

What are the three factors that determine whether pathogenesis will result?

Answer 8

• pathogen
• host
• environment

Eg – ross river virus has many virulence factors (pathogen), occurs during mosquito breeding times which is influenced by climate (environment) and caused by host’s immune response (host).

Question 9

What is the most likely cause of osteomyelitis (infection of bone – bone pain + fever)?

Answer 9

Staph. Aureus

Question 10

What investigations would you do into potential bacterial infection in spine?

Answer 10

• full blood picture (FBP)
• C-reactive protein (CRP)
• Blood cultures
• Lumbar spine x-rays

Question 11

What is the mechanism of action of the beta-lactam antibiotics?

Answer 11

They interfere with cell wall (peptidoglycan) synthesis by inhibiting enzyme transpepdidase which is responsible for the cross linking of the backbone of the peptidoglycan meshwork, therefore cell wall weakens and cell bursts from osmotic pressure.

Question 12

Describe the mechanism of resistance of MRSA strains of Staphylococcus aureus to the isoxazolyl penicillins.

Answer 12

Some staphylococci have acquired a gene (mecA) which encodes for a transpeptidase with low affinity for β-lactam antibiotics – PBP2a = MRSA.

Question 13

What is the most likely cause of cellulitis with bacteraemia?

Answer 13

Strep Pyogenes (or staph)

Question 14

What is cellulitis and Erysipelas?

Answer 14

Cellulitis – spreading of pyogenic infection involving dermis and subcutaneous tissue. It usually complicates an infected wound, ulcer or other break.

Erysipelas – more superficial spreading of infection of upper dermis and involves the superficial cutaneous lymphatics. (most are from Strep). It has sharply defined borders, obvious swelling and peau-d’orange.

Question 15

Name 3 non-suppurative complications of Streptococcus pyogenes infection.

Answer 15

• Rheumatic Fever
• Acute post-strep glomerulonephritis
• Scarlet fever

Question 16

Why do these (non-suppurative complications) follow some streptococcal infections and not others?

Answer 16

The strains of S. pyogenes that produce these infections possess virulence factors that other strains don’t have.

Question 17

What is rheumatic fever?

Answer 17

Virulent strains of S. Pyogenes infecting the pharynx. Molecular mimicry results in autoimmune reaction to many host tissues.

Question 18

What is post strep glomerulonephritis?

Answer 18

May follow infection of skin or throat with S. Pyogenes strains. Deposits immune complexes (human antibodies + strep antigens) in renal glomeruli.

Question 19

What is scarlet fever?

Answer 19

Some S. Pyogenes that infect pharynx carry genes for a particular toxin – phyrogenic ecotoxin. It is a superantigen exotoxin.

Question 20

What are the shapes of bacteria?

Answer 20

• Circle (coccus) – chains, clusters, diplococcic
• Rod (bacillus) – chains, none (random)
• Filamentous
• Rigid spirals, flexible spirals
• Comma-shaped rods
• Pleomorphic

Question 21

What are the possible features of a bacteria cell?

Answer 21

• plasma membrane used for transport, nutrients, waste, respiration (no mitochondria)
• cell wall – shape and protection
• ribosomes
• nucleoid
• Capsule and slime layers – resistance to phagocytosis, adherence to surfaces
• Fimbriae and pili – adherence, DNA transfer, twitching, gliding. Thinner than flagella, many mediate attachment.
• Flagella – swimming and swarming. Polar (one or both ends, vs peritrichous – spread over whole surface)
• Endospore – survival under harsh conditions. Very resistant thick-walled cells that develop within some species. May remain viable for 100,000 years. Contain the chromosome in their chore.
• Plasmids – small, circular, supercoiled DNA which replicated independently of host chromosome, and carries genes not essential to host but may be useful (i.e. R plasmids encode for antibiotic resistance)

Question 22

What are the 3 main mechanisms of horizontal gene transfer?

Answer 22

a) Conjugation - contact
b) Transduction – mediated by bacteriophages (viruses)
c) Transformation – free DNA

NOTE: reason antibiotic resistance spreads rapidly through populations

Question 23

How do bacteria grow?

Answer 23

Binary Fission

a) young cell
b) chromosome replicates
c) septum begins to grow
d) septum is synthesized – daughter cells are divided

Bacteria increase is logarithmic. The generation time (g) = time for a cell to produce 2 cells (population to double in number). 
BUT in reality:
-	nutrients are used up
-	metabolic waste accumulates
-	space is limited
-	oxygen depletion

Question 24

How are bacteria classified in relation to oxygen?

Answer 24

1. obligate arobe
2. facultative anaerobe
3. obligate anaerobe

Question 25

How are bacteria classified in relation to temperature?

Answer 25

Temps:

• *mesophile (20—45) – almost all human pathogens
• psychrophile (-5 – 20)
• thermophile (40 – 80)
• hyperthermophile (65 – 113)

Question 26

How is designing cultural media for bacteria determined?

Answer 26

a) sources of energy for growth
b) diversity of chemicals needed
c) need for growth factors (i.e. vitamins)

Examples:

1. Liquid culture
2. Solid culture (agar)

Question 27

How are bacteria named?

Answer 27

Order – family – genus – species - strain

Strain – populations derived from single cells – the descendants of a single, pure, microbial culture. Strains of a species differ only slightly from one another.

Species – a collection of strains that share many stable properties and differ from other groups of strains

Use italics or underline for bacterial names, i.e. Escherichia coli. Genus starts with upper case (‘Escherichia’) and species starts will lower.