Microbiology Flashcards
What is a pathogen?
An organism that causes or is capable of causing disease.
What is a commensal bacteria?
Organism which colonises the host but causes no disease in normal circumstances.
What is an opportunist pathogen?
Microbes that only cause disease if host defences are compromised.
Define virulence/pathogenicity:
The degree to which a given organism is pathogenic.
What is asymptomatic carriage?
When a pathogen is carried harmlessly at a tissue site where it causes no disease.
The bacterial nomenclature includes:
Genus and species i.e. Staphylococcus aureus.
What areas are open to bacterial colonisation?
Mucosal surfaces
Bacterial classification is based on:
Morphology: coccus, bacillus (rod)
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Gram stain: positive (purple) and negative (pink)
Cocci can be:
Diplococcus: in pairs
Chain of cocci
Cluster of cocci
Rods can be:
Chain of rods
Curve rod (vibrio)
Spiral rod (spirochaete)
What stains are used in the Gram stain?
Crystal violet
Iodine
Decolourisation
Counter stain with saffranin
What temperature range do bacteria survive?
<-80 to +80 degrees (120 Celsius for spores)
What pH range do bacteria live in?
<4-9
How long can bacteria live without water?
2 hours - 3 months (>50 years for spores)
Can bacteria survive UV light?
Yes - some species.
Growth rate of viruses
Most viruses need cells and doubling time <1 hour.
Growth rate of E. coli and S. aureus
Needs broth or solid media, doubling time takes 20-30 mins.
Growth rate of Mycobacterium tuberculosis
Needs broth or media, doubling time takes 24 hours.
Growth rate of fungi (Candida albicans)
Needs broth or media, doubling time takes 30 mins.
Growth rate of Mycobacterium leprae.
Needs borth or media, doubling time 2 weeks.
Endotoxin
Component of the outer membrane of bacteria, e.g. LPS in Gram negative bacteria.
It has a non-specific action, stable to heat, weak antigenicity and is not convertible to toxoid.
Exotoxin
Secreted proteins of Gram positive and Gram negative bacteria. I.e. Tetanus toxin
It is a protein, with specific action, labile to heat, has strong antigenicity and can be converted to toxoid.
Bacterial DNA
ds DNA
Where does transcription and translation take place in bacteria?
In the cytosol (30S and 50S ribosomes)
Bacterial genetic variation is due to mutation:
Base substitution, deletion or insertion.
Gene transfer
Transformation via plasmid
- Promotion genes
- Antibiotic and virulence determinant genes
- Maintenance genes
Transduction via phage.
Conjugation via sex pilus.
Examples of obligate intracellular bacteria?
Rickettsia spp.
Chlamydia spp.
Coxiella
MacConkey Agar is used for
Enteric bacteria
Bacteria that can be cultured on artificial media can be grouped based on:
Cell wall.
The one with no cell wall: mollicutes spp. (Mycoplasma pneumoniae)
What are the groups of bacteria belonging to With a cell wall and growing as filaments?
Actinomyces
Nocardia
Streptomyces
Bacteria with cell wall and growing as single cells:
Rods
Cocci
Spirochaetes
What bacteria belong to spirochaetes?
Leptospira
Treponema
Borrelia
Gram negative cocci is grouped as:
Anaerobic: Veillonella
Aerobic: Neisseria (N. meningitidis; gonorrhoeae)
Gram positive cocci is grouped as:
Aerobic: Staphylococcus and Streptococcus.
Anaerobic: Peptostreptococcus
Streptoccus can be:
Beta haemolytic: S. pyogenes
Alpha haemolytic S. pneumoniae and oralis
Non-haemolytic S. bovic.
Enterococcus (E. faecalis)
Lancefield group
Beta haemolytic streptococci are further differentiated / classified according to the properties of the antigens on their cell wall and given letters of the alphabet. This is known as the Lancefield grouping, hence ‘group A streptococci’ as an alternative name for S. pyogenes.
Rods - Zielh-Neelsen stain positive
Mycobacteria
Gram positive rods can be
Anaerobic: Clostridium spp and Propionibaterium spp.
Aerobic: Corynebacterium spp, Listeria spp, Bacillus spp.
Gram negative rods
Anaerobic: Bacteroides
Aerobic: Vibrio, Campylobacter, Helicobacter, Haemophilus, Brucella, Bordetella, Pasteurella
Facultative anaerobic: Coliforms (Escherichia, Klebsiella, Salmonella, Shigella, Citrobacter , Proteus, Yersinia, Pseudomonas
