Introduction to microbiology PSP1

virus

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

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 cell Cause cell lysis- viral DNA replicated Remain latent within the cell , divide with the cells natural division , virus DNA is replicated during this division 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 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.

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

naming Bacteria are named with 2 words ( genus + species) Staphylococcus aureus shortened to S.aureus

morphology Sphere ( cocci) Rods ( bacilli) Spiral (spirochaetes) Staphylococcus – spheres bunch together Streptococci- spheres form a chain Rods ( bacilli) Spiral (spirochaetes)

Gram staining 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.

Positive and negative 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.

Growth requirements 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

Cultured on an agar plate

Staphylococcus aureus 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

Pseudemonas aeruginosa Common wound pathogen Gram negative Pyocyanin – green pigment secreted by the bacteria One 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 There are also group B strep and group C but these are not comon wound infectors

Streptococcus pyogenes Strep throat Purulent infections cellulitis impetigo necrotising fasciitis ( flesh eating) Can lead to toxic shock

group G Strep 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

f FUNGI

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

dermatophytes affecting the skin and nails 3 genera Trichophyton Epidermophyton Microsporum all endemic to communal areas

Pathology First result Potassium hydroxide wet mount – dissolves Keratin leaving resistant Fungal hyphae- seen through microscope Second result Fungal culture- agar plate Therefore 2 weeks for results to be returned from the pathology laboratory

Tricophyton rubrum