Classification of Pathogenic Bacteria Flashcards
How do bacteria reproduce?
Binary fission
What is lipid A?
Endotoxin, component of LPS
Whats the difference between gram positive and gram negative bacterial cell walls?
Gram positive - peptidoglycan layer on outside, stains purple under gram staining
Gram negative - peptidoglycan layer is found underneath the LPS layer, stains pink under gram staining
What is gram staining?
- Gram staining differentiates bacteria by the chemical and physical properties of their cell walls by detecting peptidoglycan, which is present in the cell wall of gram-positive bacteria.
- Gram-positive bacteria retain the crystal violet dye, and thus are stained violet, while the gram-negative bacteria do not; after washing, a counterstain is added (commonly safranin or fuchsine) that will stain these gram-negative bacteria a pink colour.
Why is gram staining useful?
Allows us to classify organisms
Different classes of antibiotics work on each
Gives an early indication of cause of infection
What are the different forms of cocci bacteria?
Cocci (single round organism)
Streptococci (string)
Staphyococci (clusters)
What different shapes do bacteria come in?
Cocci, bacilli and spirochaete
What is the coagulase test used for?
Coagulase test is used to differentiate Staphylococcus aureus (positive) from Coagulase Negative Staphylococcus (CONS). Coagulase is an enzyme produced by S. aureus that converts (soluble) fibrinogen in plasma to (insoluble) fibrin. If the staphylococcus is coagulase positive, the plasma will clump. If no clumping occurs, the staphylococcus is coagulase negative.
What test is used to differentiate between staphylococci species?
Coagulase test (positive clumps, negative doesn’t)
Name a coagulase positive staphylococci
Staphylococci aureus, usually commensal antigen on skin and mucosal surfaces but can be pathogenic. Produces two super antigens. Can cause boils, skin infections, septicaemia and food poisoning. B-lactamase resistance has developed, as well as MRSA (methicillin resistant SA).
Name a coagulase negative staphylococci
Staphylococci epidermidis - another example of a skin commensal that can be pathogenic when it enters wounds for example found on foreign objects in body, surgery to put in prostheses, in the immunocomprimised etc.
What are the classifications of different species streptococci based on?
Haemolytic properties (their effect on a blood agar plate)
- If it converts red agar to green due to the oxidation of iron in haemoglobin, they are alpha-haemolytic
- If it converts agar to a clear colour, they are beta haemolytic
- no change - non-haemolytic
Name some examples of alpha-haemolytic streptococci
Streptococci pneumoniae - pneumonia, meningitis, septicaemia
Streptococci viridans -normal oral flora, infective endocarditis
Names some examples of beta-haemolytic streptococci
Streptococcus progenies (group A, sore throats, cellulitis, necrotising fascitis) Streptococcus agalactiae (group B, neonatal sepsis and meningitis) Enterococcus spp. (group D now, commensal, UTI)
How are beta haemolytic streptococci further subdivided?
Based on surface antigens, according to Lancefield groupings
A-G, though A, B and D are the most important clinically
Describe the clostridium species of bacteria
Anaerobic gram positive bacilli
Name some members of the clostridium spp
Clostridium tetani - tetanus
Clostridium perfringens - soil and normal commensal in gut, though can infect wounds and cause gas gangrene
Clostridium difficile - commensal, opportunistic during antibiotic usage to cause diarrhoea and pseudomembranous colitis
Clostridium botulinum - paralysis due to toxin, botox
Describe the Neisseria spp
Gram negative aerobic bacilli
What type of bacterium is Moraxella catarrhalis?
Gram negative bacilli that causes respiratory infections, especially in those with underlying lung pathology
What are coliforms and how can they be subdivided?
- Coliform bacteria are defined as rod-shaped Gram-negative non-spore forming and motile or non-motile bacteria which can ferment lactose with the production of acid and gas when incubated at 35–37°C.
- Mostly inhabitants of the human gut
- Lactose fermentation on MacConkey agar is a useful preliminary test in identifying Gram negative bacilli
What do the results of a MacConkey agar test tell you?
- MacConkey agar is a selective and differential culture medium for bacteria designed to selectively isolate Gram-negative and enteric (normally found in the intestinal tract) bacilli and differentiate them based on lactose fermentation.
- By utilizing the lactose available in the medium, Lac+ bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid, which lowers the pH of the agar below 6.8 and results in the appearance of pink colonies.
- Non-Lactose fermenting bacteria such as Salmonella, Proteus species, Yersinia, Pseudomonas aeruginosa and Shigella cannot utilize lactose, and will use peptone instead. This forms ammonia, which raises the pH of the agar, and leads to the formation of white/colourless colonies on the petri plate.
Is E.Coli a lactose fermenter?
Yes - turns agar pink
Describe the Escherichia Coli bacterium
Gram negative lactose fermenting aerobic bacilli which commonly cause UTIs, diarrhoea and haemolytic uraemic syndrome
Whats the common causative agent of travellers diarrhoea?
Enterotoxogenic E.Coli
What causes haemolytic uraemia syndrome?
Enterohaemorrhagic E.Coli (E.Coli 0157)
What are the 160+ serotypes of E.Coli based on?
Their O antigens (LPS)
Describe briefly some virulence mechanisms of E.Coli
Pili
Capsule
Endotoxins and exotoxins
Is Salmonella a lactose fermenter?
No - turns MacConkey agar clear
Is Shigella a lactose fermenter?
No - turns MacConkey agar clear
Describe Salmonella spp., including some examples
Gram negative aerobic non lactose fermenter bacilli
Salmonella enterica - >1500 serotypes, causes self-limiting enterocolitis with or without bloody diarrhoea
Salmonella typhi - typhoid fever (constipation and fever)
What is the 2nd most common cause of bacterial diarrhoea?
Salmonella enterica
Give some examples of coliforms
E.Coli spp. Klabsiella spp. Proteus spp. Salmonella spp. Shigella spp.
What type of bacterium is Shigella spp?
Gram negative aerobic large coliform bacilli, non lactose fermenter
What is typical of a Shigella spp. infection?
Diarrhoea and Dysentery
Describe the Klebsiella spp. and what is typical of its infection
Gut commensal, gram negative aerobic coliform, involved in UTIs and pneumonia
Describe the proteus spp. and what is typical of its infection
Gut commensal, gram negativeaerobic coliform, causes UTIs often associated with stones
What bacterium species is a curved gram negative bacilli?
Campylobacter spp.
What conditions are suitable for the growth of campylobacter and helicobacter species?
Microaerophilic
Whats the usual source of campylobacter?
Domestic animals e.g. chickens, faecal-oral route
What is typical of a campylobacter infection?
Smelly bloody diarrhoea
What is the most common cause of bacterial diarrhoea in the UK?
Campylobacter spp.
What does helicobacter look like under the microscope?
Spiral bacilli
Describe helicobacter pylori and the results of its infection
Gram negative microaerophilic spiral bacilli normally found in the stomach, which can damage the mucosa to lead to the formation of gastric ulcers and cancer
Describe Haemophilus influenzae and the result of its infection
Gram negative aerobic small bacilli which causes respiratory infections, which can exacerbate COPD. Its capsulate form is important in meningitis
Describe the Pseudomonas species and its infection
Gram negative strictly aerobic bacilli usually found in water and soil. Causes hospital caused sepsis, pneumonia, UTIs and respiratory infections in CF patients.
Name some anaerobic gram negative species of bacteria
- Bacteriodes spp (colonic)
- Prevotella (oral)
- Porphyromonas (oral)
Describe bacteriodes fragilis and its infection
Anaerobic gram negative bacilli, part of normal colonic flora. Can cause intra-abdominal abscesses
What are some examples of gram negative oral anaerobic bacteria?
- Prevotella
- Porphyromonas
- Pasteurella spp
- Capnocytophyga spp
Name some species of bacterium that do not belong to the gram negative or gram positive groups
– Mycobacterium species
– Spirochaetes
– Chlamydia / Chlamydophila
Describe the Mycobacterium species of bacteria
Aerobic acid and alcohol fast bacilli, grow in mould like fashion
What stain is used to identify the Mycobacterium species?
- Mycobacterium species are seen using the Ziehl-Neelsen (ZN) Stain
- The Ziehl–Neelsen stain, also known as the acid-fast stain is a special bacteriological stain used to identify acid-fast organisms, mainly Mycobacteria.
- Acid fast organisms like Mycobacterium contain large amounts of lipid substances within their cell walls called mycolic acids. These acids resist staining by ordinary methods such as a Gram stain. It can also be used to stain a few other bacteria, such as Nocardia. The reagents used are Ziehl–Neelsen carbol fuchsin, acid alcohol, and methylene blue. Acid-fast bacilli will be bright red after staining.
What bacteria causes tuberculosis?
Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis (MTB).
What bacteria causes leprosy?
Mycobacterium leprae – cannot be cultured
What species of bacteria encompasses the bacteria that cause both tuberculosis and leprosy?
Mycobacterium spp
Describe a spirochaete
Anaerobic, long spiral shaped bacteria with two membranes. Were thought to be gram negative but some say the wall is too thin to be stained.
What are the following examples of?
Treponema pallidum
Borrelia burgdorferi
Leptospira interrogans
Spirochaete spp.
What organism causes syphilis?
Treponema pallidum (spirochaete)
Describe the different types and stages of a syphilis infection
- Primary syphilis: non-painful skin lesion (chancre) at the site of infection (skin or mucous membranes)
- Secondary syphilis (6-8 weeks after primary symptoms): generalised systemic illness and rash
- Latent phase: symptomatic episodes may occur
- Tertiary syphilis (years after primary symptoms): central nervous system
- Congenital syphilis: stillbirth, neonatal death or disease
What organism causes Lyme disease?
Borrelia burgdorferi
What is erythema chronicum migrans?
Characteristic “ bulls eye” rash seen in Lyme disease infection
Describe the three stages of Lyme disease infection
- Stage 1: Skin rash (erythema chronicum migrans) “red and white target” appears at the site of the tick bite
- Stage 2: Systemic illness occurs in some patients weeks or months later when patients suffer cardiac or neurological and musculoskeletal symptoms
- Stage 3: Chronic disease, occurring years later when patients present with chronic skin, nervous system or joint abnormalities
What organism causes Chlamydia?
Chlamydia/Chlamydophila spp - group of aerobic obligate intracellular bacteria with a unique developmental cycle (similar to spore formation in fungi). Many unusual plant like features
Describe some members of the Chlamydia species
Chlamydophila pneumoniae - respiratory infection (RI)
Chlamydophila psittaci - contact with birds, RI
Chlamydophila trachomatis - trachoma, genital and neonatal infections. Often asymptomatic in women, can lead to pelvic inflammatory disease and infertility.
