Introduction to microbiology

PSP1

virus

• Smallest known infective agent• Most forms of life a susceptible to viral infection-

humans, animals, plants, bacteria• Need a host to replicate• Requires entry to the host

– Respiratory tract– Alimentary tract ( oral –faecal route)– Blood– Sexual intercourse– Skin abrasions

virus

• Basically :• a core of nucleic acid ( contains genetic

material DNA/RNA) surrounded by a protein coat

• Outer protein envelope

replication• Often protein envelope partially derived from

the host cell• Virus always replicate INSIDE a host cell• Fragile outside a host cell1. Cause cell lysis- viral DNA replicated2. Remain latent within the cell , divide with the

cells natural division , virus DNA is replicated during this division

1. THEN cause lysis

• Different virus attack different cells depending on the RECEPTOR SITE

• Systemic- spreads widely and infects many tissues and organs e.g. measles

• Localised- invades only tissues adjacent to the site of entry e.g respiratory viral infections/ verrucae

• Main defence of the body is to produce the protein interferon

• Interferon is released by infected cells and taken up by other cells

• Antibodies appear in the patient's serum- this leads to immunity or resistance

• Hepatitis A, Hepatitis B, and Hepatitis C are diseases caused by three different viruses

• different modes of transmission and can affect the liver differently.– Hep A- ingesting faecal matter ( often associated with

developing countries/travel)– Hep B- blood/body fluid– Hep C- blood/body fluid

• There are vaccines to prevent Hepatitis A and B;

however, there is not one for Hepatitis C.

Hepatitis

HIV• Human immunodeficiency virus• AIDS- final stage of HIV infection• 1 in 5 affected people do not know they

have the virus• Transmitted through:

– Unprotected sex– Mother-baby– Contaminated needles

bacteria

• Single cell organisms• Classified in different ways ( three

parameters): • Shape- morphology• Colour ( Gram staining)• Growth requirements-Need for oxygen

• Bacteria are named with 2 words ( genus + species)

• Staphylococcus aureus shortened to S.aureus

naming

• Sphere ( cocci)– Staphylococcus – spheres bunch together– Streptococci- spheres form a chain

• Rods ( bacilli)• Spiral (spirochaetes)

morphology

• Gram positive and gram negative refers to how a bacteria reacts to a gram stain.

• Chrystal violet then iodine• If it takes the initial stain, it will be purple and

be considered gram positive.• If it doesn't take the initial stain, it will be pink

and gram negative.

Gram staining

• The difference is the outer casing of the bacteria.

• A gram positive bacteria will have a thicker layer of peptidoglycan (a sugar-protein shell)

• A gram negative bacteria has an outer membrane covering a thin layer of peptidoglycan on the outside.

Positive and negative

• Aerobes need oxygen and are found on wound surfaces– E.g. Pseudomona aeruginosa

• Anaerobes cannot survive where there is oxygen and are found deep in wounds – E.g. Clostridium welchii – causes gas gangrene

Growth requirements

Cultured on an agar plate

• Gram positive• Staphylococcus aureus is a bacterium that

commonly colonises human skin and mucosa (e.g. inside the nose) without causing any problems. Normal body flora

• Common wound infector- may lead to cellulitis

• Methicillin Resistant Staphylococcus aureus

Staphylococcus aureus

Pseudemonas aeruginosa

Common wound pathogenGram negativePyocyanin – green pigment secreted by the bacteriaOne of the most worrisome characteristics of P. aeruginosa is its low susceptibility to antibiotics

Streptococcus pyogenes ( group A strep)•Gram positive•Faculative anerobe ( can survive with or without oxygen)•Part of normal flora in many people’s throats•Can cause simple infections ( sore throat) to life threatening infections•Most frequent pathogen in humans

• Strep throat• Purulent infections• cellulitis• impetigo• necrotising fasciitis

( flesh eating)• Can lead to toxic

shock

Streptococcus pyogenes

• MAY be found as part of normal skin flora

• Normally infections in patients with other co-morbidities e.g diabetes

• Wound infector• Can spread and

cause extensive infections: e.g.

• Bacteriamia• Septic shock

group G Strep

FUNGIf

Fungal hyphae

Multicellular fungi are composed of filaments called ‘hyphae’

• More complex organism than bacteria• Reproduce by spore formation ( released by

splitting hyphae) or sexual reproduction ( mix chromosomes)

• Cell wall+ cell membrane• Ergesterol essential element of the cell

membrane – provides stability and flexibility

fungi on skin and nails

DERMATOPHYTES infect keratinous material NON-DERMATOPHYTES Yeasts or moulds

Fungi cannot manufacture their own food. Dermatophytes ingest keratin as their food source

• 3 genera– Trichophyton– Epidermophyton– Microsporum

• all endemic to communal areas

dermatophytes affecting the skin and nails

Pathology

First resultPotassium hydroxide wet mount – dissolves

Keratin leaving resistant Fungal hyphae- seen through microscope

Second resultFungal culture- agar plate

Therefore 2 weeks for results to be returned from the pathology laboratory

Tricophyton rubrum