Identify bacteria by gram staining Flashcards
Which bacteria stain poorly/ don’t stain with Gram stain
Chlamydia (intracellular bacteria)
Coxiella
Legionella
Mycobacterium
Mycoplasma (no cell wall)
Rickettsia
Spiral - treponema, leptospira, borrelia
Gram positive bacteria divided into cocci and bacilli
How are cocci further divided?
Cells in clusters/ catalase positive - Staphylococcus
Cells in pairs/chains / catalase negative - streptococcus
How are staphylococci further characterised?
Coagulase positive - staph aureus
Coagulase negative -
- Staph epidermidis
- Staph saprophyticus
- Staph capitis
Coagulase is an enzyme which causes clotting, by converting fibrinogen to fibrin
What is role of coagulase enzyme in staph aureus
Coagulase reacts with prothrombin in the blood producing complex called staphylothrombin
This enables the enzyme protease (from liver) to convert fibrinogen to fibrin.
This results in clotting of the blood.
Coagulase is tightly bound to the surface of the bacterium S. aureus and can coat its surface with fibrin upon contact with blood. The fibrin clot may protect the bacterium from phagocytosis and isolate it from other defenses of the host.
What is role of catalase enzyme in identifying gram positive bacteria
Catalase converts hydrogen peroxide to oxygen and water, thereby preventing oxidative damage.
We test to see if catalase positive, as can help (in addition to other tests) in identifying bacteria
Streptococci are further classified by their haemolytic properties on blood agar.
What effect do with gamma, alpha and beta haemolytic strep have on blood agar
Gamma-hemolytic species cause no hemolysis (also known as non-haemolytic)
Alpha-hemolytic species cause oxidization of iron in hemoglobin molecules within red blood cells, giving it a greenish color on blood agar
Beta-hemolytic species cause complete rupture of red blood cells. On blood agar, this appears as wide areas clear of blood cells surrounding bacterial colonies
What are examples of -
- non-haemolytic strep
- alpha haemolytic strep
- gamma haemolytic strep
Strep milleri
Anaerobic strep
Some enterococci - alpha haemolytic strep
Strep pneumoniae
Strep viridans (viride is green in latin)
Beta-hemolytic streptococci are further classified by Lancefield grouping, a serotype classification (that is, describing specific carbohydrates present on the bacterial cell wall) The 20 described serotypes are named Lancefield groups A to V (excluding I and J). H-S are non-pathogenic to humans
What are examples of -
Group A, B, C, streptococci
(these are main groups which cause human infection)
Group A -
Strep pyogenes - means produces pus
Group B - means unable produce milk. Infection neonatal/ maternal
Strep agalactiae
Group C - these are zoonoses Strep dysgalactiae - difficulty producing milk Strep equi - from horses Strep equisimillis - horse-like Strep zooepidemicus - outbreaks in zoos
What are examples of -
Group D, F, G streptococci
these are main groups which cause human infection
Group D - orginate in bowels
Enterococcus faecalis
Enterococcus faecium
Group F - (previously called strep milleri group
Strep anginosus
Strep intermedius
Strep constellatus
Group G -
Strep dysgalactiae - exact same bacteria as Group C version, except expressing different antigen - G
Out of all gram positive bacteria, which ones are most medically relevant?
Staph aureus
Alpha haemolytic -
Strep pneumoniae
Strep viridans
Beta haemolytic -
Group A - strep pyogenes
Group B - strep agalactiae
Beta-haemolytic strep A, C, G, can be thought of together, because they have similar sites of infection
Where in normal flora do A, C, G live?
What infections do they cause?
A - mucus membranes
C - mucus membranes and animals
G - mucus membranes and bowel (cancer)
Tonsillitis
Cellulitis
Septic arthritis
Osteomyelitis
Beta-haemolytic strep B, D, F can be thought of together, because they have similar sites of infection
Where in normal flora do B, D, F live?
What infections do they cause?
B - bowel, female genital tract
D - bowel
F - bowel
B - neonatal sepsis, septic arthritis, IE, infections in diabetes, associated pelvic cancer
D - IE, peritonitis, IV/ urinary catheter infection
F - peritonitis, empyema
Where does staph aureus live
What infections does staph aureus cause
Skin
Skin Bone IE Pulmonary - emypema/ pneumonia Device-related infections
What infections do coagulase negative staph cause e.g staph epidermidis/ capitis/ hominis/ saprophyticus
S epidermidis - skin infections primarily, but also causes infections associated with implants - .eg IE, VP shunt, artificial joints
S capitis - skin
S hominis - skin
S saprophytocis - can cause UTI
Gram negatives divided into cocci and bacilli
Cocci are further divided into diplococci, coccobacilli, comma shaped, spirochete
What are examples of diplococci?
Neisseria Mengitidis - maltose fermenter
Neisseria Gonorrhoae - non-maltose fermenter
Moraxella catarrhalis
Lives mucus membranes
Gram negative bacilli can be divided into aerobic/ anaerobic/ facultative/ microaerophile
What are examples of anaerobic bacteria
Bacteroides
Lives as normal gut flora
Cause intra-abdominal infections related to perforation/ GI surgery
Fusobacterium
Cause Lemierre’s syndrome/ oropharyngeal disease
When looking at gram stains, look for field with neutrophils, as most likely to indicate bacteria causing infection, as opposed to commensal.
If from sterile site - should only see one morphotype of batceria
What is identification based on gram stain and morphologic feature, and what is most likely site -
- gram positive cocci in clusters/ treads
- gram positive cocci in long chains
Staphylococci
GI, respiratory tract, skin, blood, urine
Streptococci - all groups including strep viridans and enterococci
What is identification based on gram stain and morphologic feature, and what is most likely site -
- gram positive diplococci and chains occassionally with a capsule
Strep pneumoniae
Respiratory, blood, CSF
Gram positive diplococci differ from gram negative dpilococci in that former are pointed at the ends, and latter are kidney bean shaped
