Bacteria, Positive and Negative Grams Flashcards
Define a pathogen
A disease causing micro-organism
Define a commensal
Organism which colonises the host but doesn’t cause disease
Define virulence
Degree to which an organism is pathogenic
Define opportunist pathogen
Microbe that causes disease only if host is compromised
Define an asymptomatic carriage
Pathogen harmlessly carried at tissue site that doesn’t cause disease
What is the hierarchy of microbial size?
Protozoa –> Bacteria –> Viruses
How do you tell the difference between gram positive and negative bacteria including stain?
Positive: Thick peptidoglycan coating with single membrane, stains purple. Can also form spores
Negative: Thin peptidoglycan coating, double membrane carrying endotoxin
What organism is the Ziehl-Neesen used to stain for?
Mycobacteria
How is the stain carried out?
1) Apply crystal violet to heat fixed bacteria
2) Treat with iodine
3) Decolourise sample
4) Counterstain
What environmental factors affect bacterial growth?
Temp: -80 –> +80 degrees
pH: <4 - 9
Water/Desiccation
Light: UV
What are the 3 phases of bacterial growth?
Lag
Exponential
Stationary
What are 2 functions of pili and function of flagelli?
Pili: a) Plasmid exchange b) Adhere to surfaces
Flagelli: Locomotion propulsion
Which bacteria release exo and endo toxins?
Exotoxins: Both gram positive and negative
Endotoxins: Gram negative
What are endo and exotoxins?
En: Outer membrane component released when bacteria damaged (Heat stable, toxic to host and less specific)
Exo: Specific and heat labile proteins secreted by gram positive and negative
What are plasmids special ability?
Circular fragments of DNA that are able to carry genes for antibiotic resistance
How are staphylococci and streptococci arranged?
Sta: Clusters of Cocci
Str: Chains of Cocci (S Aureus –> G +ve)
What is the chemical test to distinguish the two types of gram positive bacteria?
Catalase Test: Detects for presence of catalase enzyme using hydrogen
peroxide
a) Sta: Cat +ve b) Str: Cat -ve
How do you distinguish further between the 2 sub types of bacteria:
Str: Blood Agar haemolysis (Serougrouping done if in B haemolysis group)
Sta: Coagulase Test: See if a fibrin clot has been produced
Where is staphylococcus found, spread and virulence factors?
Found: Nose and Skin
Spread: Aerosols and Touch
VF: Toxins, Proteases, Toxic shock syndrome toxin, Protein A
What are the 3 shapes of single cell bacteria?
1) Rod (Bacillus)
2) Cocci (+ve are Str and Sta)
3) Spirochetes
How is bacterial variation brought about?
1) Mutation: Del, Base Sub, Ins
2) Gene Tech: Transformation, Translation and Conjugation
What are examples of gram negative and positive bacilli?
-: Shigella, salmonella, E. Coli, etc
+: Clostridium, bacillium, Corynebacterium
What bacteria uses MacKonkey agar?
Gram negative bacilli
What is found in MacKonkey agar?
Bile Salts, Lactose, pH indicator. (If lactose produced by organism, lactic acid produced and agar will be red/pink)
What are 2 gram negative bacteria that produce a positive result for MacKonkey agar?
1) E. Coli
2) Klebsiella pneumoniae
How does c.diptheriae spread?
Droplet
Does shigella have an H antigen?
Has no flagellum as non-mobile therefore has no H antigen
No MacKonkey result as doesn’t ferment lactose
Does salmonella contain an H antigen?
Salmonella motile, so has flagellum and an H antigen
No MacKonkey as doesn’t ferment
Does E. Coli produce a positive MacKonkey agar result?
Ferments lactose –> Red/pink result (+ve)
Contains flagellum so motile and H antigen
How do you detect differences between the different gram negative bacilli?
MacKonkey agar and Serology to detect presence of H antigen
Why does E.Coli have pathogenic strains?
Due to recombinant strands of DNA from acquisition of other bacteria
How does ETEC cause traveller’s diarrhoea?
1) ETEC heat labile toxin modifies Gs protein in “locked on” state
2) Adenylate cyclase activated so increased cAMP production
3) Increased Cl- secretion in the intestinal lumen
4) H2O follows down osmotic gradient causing traveller’s diarrhoea
What effects do EPEC and EHEC e.coli have on mucosa?
Adhere to microvilli, rearrange actin leading to pedestal formation
EPEC: Chronic water diarrhoea
EHEC: Bloody diarrhoea
What is the transmission, symptoms, AT advantage and action of Shigella?
T: Contaminated food/water, or person:person
S: Frequent passage, severe bloody diarrhoea
AT: Moves through V stomach pH without being destoryed and can infect the stomach
A: Induction of self uptake > Apoptosis > Cytokines released and neutrophils attracted causing inflammation then occurs in adjacent cells
What bacteria causes salmonella and what are the 3 types of salmonella?
Bacteria: S. enterica
1) Gastroenteritis
2) Enteric fever
3) Bacteraemia
What are gastroenteritis and enteric fever?
G: Freq. cause of food poisoning w/ 24 hr incubation period -> Highly infective dose
E: Typhoid fever (Systemic dose)
What are the symptoms and dangers of v. cholerae?
S: Huge volumes of watery stools
D: Large amounts of water loss leads to hypovolemic and severe dehydration
- Grows at 18-42 degrees so withstands fever
- 8 alkaline pH sensitive so sensitive to the pH of the stomach
How would haemophilus influenzae grow and what diseases does it cause?
Grow: Chocolate Agar (Needs Haem and NAD)
Diseases: Meningitis and Pneumonia
What happens in the pathogenesis of gastro-enteritis?
1) Endocytosis
2) Chemokine release
3) Neutrophil recruitment and migration
4) Neutrophil induced tissue injury
5) Fluid and electrolyte loss –> Diarrhoea
What happens in pathogenesis of enteric-fever?
1) Endocytosis
2) Migration to basolateral membrane
3) Survival in macrophage –> Systemic Spread
What disease does Legionella cause?
Legionnaires disease (Immunocompromised Individuals)
What type of bacteria are Neisseria? And what are 2 examples?
Gram negative diplocci
N. meningitidis and N. gonorrhoeae
What is the transmission, pathogenesis and virulence of N.meningitidis?
T: Aerosol, with high risk in colonised
P: 1) Cross N-P epithelium and cross blood stream
2) Asymptomatic bacteraemia or septicaemia
V: Capsule (A-P), LPS and Pili (adherence to host cell)
What are bacteroides?
Opportunistic, obligate anaerobes
What are the consequences of N.gonorrhoeae infection?
STI - rectal, vaginal or oral inflammation.
How is Chlamydia detected and how is it grown?
D: Serum antibodies/PCR
G: Not grown as an intracellular obligate parasite
What are the 2 developmental stages of chlamydia’s unique growth cycle?
1) Elementary bodies (infective).
2) Reticulate bodies (intracellular multiplication).
(Reticulate bodies are converted back into elementary bodies and are released. The cycle continues)
What are 2 bacteria found in the chlamydophila genre?
1) C.pneumoniae - respiratory tract infection.
2) C.psittaci - associated with birds.
What sort of flagellum do spirochaete have?
Endoflagellum: Found between I and E membrane
What spirochaete causes:
a) Lyme Disease
b) Syphillis
a) B. burgdoferi
b) T. pallidum
What are the 3 stages of syphillis?
- Primary stage: localised infection.
- Secondary stage: systemic - skin, lymph nodes etc.
- Tertiary stage: CV syphilis and neuro syphilis.
