introduction to medical microbiology Flashcards
list the categories of microorganism that cause infection
bacteria, viruses, fungi, parasites, prions
bacteria
prokaryotic; round, spiral or rod shaped single celled prokaryotic organism that typically lives in soil, water, organic matter or the bodies of plants and animals ; 0.2-0.5 μm
viruses
unclassified; unique, acellular, metabolically inert organism that only replicate within living cells; 20-400 nm
fungi
eukaryotic; any of a kingdom of saprophytic and parasitic spore-producing eukaryotic typically filamentous organisms including moulds, yeasts and mushrooms; 2-10 μm1
parasites
eukaryotic (can be vector for prokaryotes); an organism living in, with or on another organism; microscopic to over 30m
prions
unclassified; protein of unknown function that resides on the surface of brain cells, an abnormal conformational form of prion protein that in mammals includes pathogenic forms that arise spontaneously or transmission and upon accumulation in the brain cause a prion disease; 10nm
what are the two main types of specimins
sterile and non sterile
what is a sterile site
organisms detected here are likely to be clinically significant ○ Blood ○ CSF ○ Bladder
examples of non-sterile sites
LUNGS: ○ Lung ○ Urethra ○ Nasopharynx ○ Gut ○ Skin
list examples of specimin collection for bacterial culture
- UTI: mid-stream urine (MSU)
- Chest infection (lower resp tract): sputum
- Tonsillitis/pharyngitis: throat swab
- Wound/site of infection: swab or pus (swabs are sent in bacterial transport medium)
- Diarrhoea: faeces
- Bacteraemia/septicaemia: blood culture
- Meningitis: CSF
- Abscess: aspirate of pus - don’t swab abscesses
Osteomyelitis: bone
how are pathogens identified using microscopy
- Light microscopy (x1000) - oil immersion lens
- Unstained: to see pus cells - WBC (urine, CSF), to see parasites (faeces)
○ Look for WBC which are indicators of infection - Gram stain: to see bacteria, yeast/fungi
- Special stains - Ziehl Neelsen stain/auramine stain: to see mycobacteria
○ Not all bacterial cells are stained with gram staining, other methods have to be used e.g. for TB
Viruses not visible in light microscope
- Unstained: to see pus cells - WBC (urine, CSF), to see parasites (faeces)
how is gram staining used in the identification of bacteria
- Rapid
- Not specific but assists provisional diagnosis and empiric antimicrobial therapy
- Can’t usually identify particular species
- Followed by bacterial culture and susceptibility testing
bacterial culture
- Slow
- Sensitive
- Culture conditions suitable for the expected species
1. Type of media: Non selected e.g. blood, Specialised e.g. mycobacteria, Selective e.g. macConkey
- Atmosphere
- Temperature
- Duration of incubation
identification of the species
- Use of observable characters ○ Morphological ○ Physiological ○ Biochemical - Rapid and simple to perform - DNA based tests - Species name e.g. Escherichia coli Typing distinguishes strains within a species for epidemiological reasons
interpretation of lab testing
- Knowledge of normal (commensal) flora at site
- Knowledge of likely pathogens at site
- Clinically significant in light of clinical picture
○ If yes, antibiotic sensitivity tests performed - Some bacteria are both commensal and pathogenic e.g. S. aureus
- Final report with interpretive comments and results
