Pathogens

Question 1

Koch’s postulates define

Answer 1

set of 4 criteria designed to establish a casual relationship between a microorganism & a disease

Question 2

what are the 4 Koch’s postulates:

Answer 2

1. the pathogenic mirco-organism must be present in every case of the disease but absent from healthy animal
2. the suspected micro-organism must be isolated & grown in pure culture
3. the same disease must occur when the isolated micro-organism is injected into healthy susceptible animals
4. the same micro-organisms must be isolated again from the injected animals which developed disease

Question 3

Exceptions of Koch’s Postulates

Answer 3

1. Tetanus
   
   1. Clostridium tetani grows in infected, anaerobic wounds, making toxins that travel to affect CNS
2. Leprosy
   
   1. Causative organism of leprosy has never been grown
   2. Accepted as the cause, being present as intracellular mycobacteria in all causes
   3. host dependent

Question 4

List major types of infections

Answer 4

1. purulent
2. bacteraemia
3. Septicaemia
4. Toxaemia
5. Granulomatous
6. Local

Question 5

Describe purulent infection

Answer 5

• pus-producing (suppurative)
• Due to the effect of bacterial leucotoxins

Question 6

example of purulent infection

Answer 6

• e.g. streptococcus spp. Corynebacterium spp

Question 7

describe Bacteraemia infection

Answer 7

temporary presence of bacteria in blood

Question 8

describe septicemia infection

Answer 8

• presence of bacteria & toxins in blood
• Invasion of the bloodstream by virulent bacteria

Question 9

e.g. septicemia infection

Answer 9

staphylococcus spp & bacillus anthracis

Question 10

Toxaemia infection & eg

Answer 10

• spread of bacterial toxins in blood
• e.g. Clostridium spp

Question 11

Granulomatous infection define

Answer 11

slowly developing granulomas form, associated with slow growing intracellular organisms

Question 12

Granulomatous infection eg

Answer 12

e.g. Mycobacterium tuberculosis, Nocardia spp, Actinomyces spp., Dermatophilus spp.

Question 13

local infection define & eg

Answer 13

• localised infection
• e.g. respiratory - Pasteurella spp, Haemophilus spp.
  
  • Enteritis - e.coli, V. cholerae

Question 14

Pathogenesis of bacterial infection

Answer 14

• pathogens have a range of attributes that promote disease in the host
• the host has a range of mechanisms to deal with an infection

Question 15

Infectious disease describe

Answer 15

• presence of a living organism in or on the animal induces abnormal function that leads to clinical signs
• implies the agent is multiplying in the host

Question 16

Define Pathogenicity

Answer 16

• ability of a species of micro-organisms to cause disease
• refers to the species as a whole

Question 17

Degrees of pathogenicity

Answer 17

• True pathogens (high pathogenicity)
  
  • e.g. salmonella
• grades of pathogenicity (low to high)
• Commensals that can become opportunists
  
  • Cause secondary infection
• Mixed infections
  
  • Cat bite abscess ( a lot of oral flora)

Question 18

Define virulence

Answer 18

• ability of a particular strain to cause disease
• degree of pathogenicity within a species
• due to certain properties or abilities possessed by that strain, which can be gained or lost

Question 19

Virulence varies with

Answer 19

Strains

Question 20

Properties of virulence

Answer 20

• can be lost/altered
• depends on the attributes if the bacteria
• colonisation/invasion/evasion/tissue damage

Question 21

Attributes of bacteria that contribute to disease

Answer 21

• must be able to colonise the surface of the host (skin, mucous membrane) often in competition with other microbes
• Be invasive
• Be able to survive or evade the host defense mechanisms
• be able to produce harm to host tissues
  
  • Toxin production
  • Immune mediated damage

Question 22

Contagious define

Answer 22

spreads directly from 1 individual to another

Question 23

Communicable disease define

Answer 23

Spreads directly or indirectly via air, water etc

Question 24

Define nosocomial infection

Answer 24

acquired in a hospital setting

Question 25

Why Pathogens need systems for:

Answer 25

- attachment to a host - Intoxication of host cells - Subverting host defences - Synchronising regulation of virulence-associated gene expression - Control the preferential secretion of virulence-associated products

Question 26

Non-specific resistance to infectious organisms: skin

Answer 26

- thick keratinised layer - For providing physical & chemical barriers to infection

Question 27

Non-specific resistance to infectious organisms: Lysozymes

Answer 27

- In body secretions - such as tears, saliva, reproductive fluids which can break down peptidoglycan

Question 28

Non-specific resistance to infectious organisms: cilia

Answer 28

- lining of respiratory tract - Beating of cilia clear organisms from the tract

Question 29

Non-specific resistance to infectious organisms: gastric acidity

Answer 29

- Down to pH 2, kills many potential pathogens - Some microorganisms e.g. Helicobacter pylori - Can live in the stomach & causes human stomach ulcers

Question 30

Non-specific resistance to infectious organisms: Motility, flow of intestinal contents, urine, mammary secretions, respiratory secretions

Answer 30

- To flush out organisms - Reduced motility or flow can lead to a build up of organisms & infection - e.g. gut stasis & blockage of urinary tract - Pathogens of these sites often have mechanisms to hold onto the epithelium so they are not flushed out

Question 31

Non-specific resistance to infectious organisms: iron binding proteins

Answer 31

e.g. lactoferrin, transferrin in milk & blood, will compete with bacteria for available iron, which the micro-organisms need for growth

Question 32

Non-specific resistance to infectious organisms: mucus

Answer 32

acts as a physical barrier, traps organisms

Question 33

Non-specific resistance to infectious organisms: Antimicrobial peptides

Answer 33

in the gut & other sites

Question 34

Non-specific resistance to infectious organisms: Normal microflora

Answer 34

- competitively inhibits growth of potential pathogens - Alteration of normal flora can allow infections - e.g. fungal infections after abx treatment which removes normal bacteria from a site

Question 35

Non-specific functions of the immune system e.g.

Answer 35

e.g. macrophages, interferons

Question 36

Site of microbial entry: skin

Answer 36

Broken allows the entry of opportunistic organisms, such as bacteria already on the skin e.g. Bacillus anthracis can enter through unbroken skin - Wet skin is softer & more susceptible to infection - e.g. to foot rot

Question 37

Site of microbial entry: Conjunctiva, mucous membranes

Answer 37

Leptospira from urine

Question 38

Site of microbial entry:Venereal (reproductive tract)

Answer 38

e.g. brucella abortus, campylobacter fetus, treponema pallidum

Question 39

Site of microbial entry: Respiratory system

Answer 39

- via pharynx, inhaled - e.g. Haemophilus spp, Pasteurella spp.

Question 40

Site of microbial entry: Ingestion

Answer 40

e.g. enteric bacteria

Question 41

Site of microbial entry: Vertical transmission

Answer 41

e.g. Salmonella, Grp B Streptococci

Question 42

Site of microbial entry: Insect vectors

Answer 42

e.g. Yersinia pestis via flea bite

Question 43

Site of microbial entry: Direct contact

Answer 43

MRSA, Leptospira, Brucella

Question 44

Describe role of transmission

Answer 44

Excretion in large numbers - e.g. respiratory or GI Carrier animals - appear healthy themselves but excrete organisms that infect other animals - need to be able to ID & remove carriers from the population to eliminate infection Resistance to environmental stress - Spores Multiple hosts, reservoirs, vectors - insect bites Vertical transmission from mother to offsprings - e.g. salmonella spp.

Question 45

Role of colonisation in bacterial pathogenesis

Answer 45

- bacteria require features to colonise target tissue, organs or systems - use various adhesion factors lead lipotechnoic acids - Some of them use proteins such as fimbriae & some produce certain protein, enabling them to attach

Question 46

describe invasion role in bacterial pathogenesis

Answer 46

- active phagocytosis by epithelial cells rather than invasion - e.g. salmonella - mechanisms for survival, multiplication & spread within & between cells - Some can spread from cell to cell - iron scavenging - e.g. siderophores

Question 47

Immune evasion role in bacterial pathogenesis

Answer 47

interfering with phagocytosis interfere with phagocytic killing Avoiding antibody or complement

Question 48

How immune evasion interfere with phagocytosis

Answer 48

- slime layer/capsule - most capsules are antigenic(antibody mediated phagocytosis) - some are not antigenic (e.g. bacillus anthracis) - prevention of fusion of lysosome & phagosome - Survive the lethal effects of the phagolysosome - Intracellular pathogens (Listeria monocytogenes, Salmonella, Mycobacteria)

Question 49

avoiding antibody or complement in immune evasion

Answer 49

- Long O-antigen side chains &/or intracellular location - Salmonella (both criteria), Mycobacteria (intracellular only)

Question 50

types of toxins produced

Answer 50

endotoxins & exotoxins

Question 51

Exotoxins produced by

Answer 51

gram - & + bacteria

Question 52

List Clostridial toxins and diseases that lead to it

Answer 52

- Tetanus (Clostridium tetani) - Botulism (Clostridium botulinum) - Enterotoxaemia (Clostridium perfringens) - Histotoxic Clostridial species

Question 53

E. coli toxins list & describe

Answer 53

- Enterotoxigenic E. coli - Heat labile enterotoxins - heat stable enterotoxins (A & B) - d+ - Shiga toxigenic e. coli - Shiga toxin (A & B) - cleaves ribosomal RNA; kills epithelial cells - cytotoxic necrotising factors - target cells & causes necrosis