Medical Mircobiology

What signs and symptoms classify a clinical infection?

Inflammation, pain, pyrexia, tachycarida, rigors, increased WBC count, increased C reactive protein

Caused by a pathogen that can invade and grow in a host

What is a pathogen?

An organisms that can cause disease

What is a commensal?

An organism which is part of normal flora (E.coli in gut)

What are Koch’s postulates?

Four criteria designed to establish a causal relationship between a microbe and a disease

1. Organism must be found in all cases of the disease

2. Able to be cultured outside the body for several generations

3. Should reproduce the disease on innoculation

How to determine if microbe is pathogen or commensal?

Use Koch’s postulates, was site sterile or non-sterlie, knowledge of normal flora site, organisms pathogenicity, clinical context

What is pathogenicity and the requirements for pathogenicity?

The capacity of a microorganism to cause an infection

Requirements:

• Infectivity - ability to become established

• Virulence - ability to cause harmful effects once established

What are the three key types of pathogens?

1. Obligate - cannot grow without spreading disease

2. Opportunistic - become pathogenic following a peturbation to their host

3. Accidental - bacteria in wrong place at wrong time and ends up causing disease

What is infectivity?

Ability to become established on or within a host

• Good attachment

• Good acid resistance

What is virulence?

Capacity to cause harmful effects when established conferred by virulence factors

• invasiveness

• Toxin production

• Evasion of immune system

What is invasiveness?

Degree to which an organism is able to spread through the body from a focus on infection

E.g. streptococcus pyogenes (necrotising fascitis)

What types of toxin can be produced?

Exotoxins - released extracellularly by the microbe

Enterotoxins - exotoxins which work on the GI tract

Endotoxin - is structurally part of the gram -ve wall (lipopolysaccharide, E.g. E.coli)

E.g. enterotoxin = cholera

What are super antigens?

Certain exotoxins of Strep P and Staph A which are able to stimulate division of T cells in the absence of a specific antigen

Cause whole families to T cells to become overproductive = cytokine storm = toxic shock

What is the gram stain?

Differentiates bacteria based on their cell wall composition

Gram -ve stain PINK

Gram +ve stain DARK PURPLE

In identifying gram +/- bacteria involvement can helo select the best class of ABs

Name some gram +ve cocci

Staphylococci (clusters)

Staph Aureus

Coagulase -ve Staphylococci

Sterptococcus pyogenes

Sterptococcus pneumoniae

a-hemolytic streptococci (blood agar green - partial haemolysis)

B-haemolytic streptococci (blood agar clear - full haemolysis)

Describe the pathogen: Clostridium Difficile

Gram +ve bacilli which can be anaerobic and aerobic

Difficult to culture and can be carried asymptomatically in the gut

Cause of diarrhoea from toxin production

Increased risk with antibiotic use/anything that disturbes normal gut flora

Detected in stool via ELISA

Name some gram -ve bacteria

Neisseria meningitidis

Coliforms from enterobacteroaceae

E.coli

Salmonella

Name some gram -ve bacilli

Campylobacter

H.Pylori

Haemophilius influenzae

Pseudonomas

What are the four types of viral infection?

Acute (common cold) _^_______

Latenet (herpes) _^_____^__

Chronic _^——————-

Tumour Virus (Hep B) ~_____________^^^^^^^^

Describe Influenza A

Virus that infects cells of the respiratory tract causing destruction of respiratory epithelium (causing 2ndy bacterial infections)

Cytokine expression = fever

Undergoes antigenic DRIFT (small changes) and antigenic SHIFT (large changes)

What are enterovirus infections?

Begin in the gut and spread to the blood (viraemia) and to other tissues

E.g. Polio

What are the twp methods of active immunisation?

Exposure and infection - stims immune response

Vaccination - promotes antigen memory

What are live attenuated vaccines?

Attenuated (weakend) pathogen promotes antibody response (virulence can occur in immunocomprimised)

Must be refrigerated

E.g. Flu nasal

What are killed vaccines?

Inactive pathogen that cannot revert virulence

Promotes weaker immune response than live so multiple doses may be required

Some contain adjuvants (to enhance response)

Possible inflammatory response as side effect

E.g. flu injection

What is a toxoid vaccine?

Modified toxin that retains its antigenicity but has no toxic activity

Only produces immunity against the toxin, not the organism producing it

E.g. tetanus