Classification of Pathogenic Bacteria Flashcards
Bacteria
- A major group of micro-organisms causing infectious disease.
- Drug resistant infections are a major global public health threat.
- Mainly single-cell prokaryotes.
- Most have a peptidoglycan cell wall
Most bacteria reproduce by
binary fission.
To classify bacteria, we can exploit differences in
cell wall structure
Gram Positive bacteria have:
- plasma membrane
- periplasmic space
- thick peptidoglycan
(inner to outer)
Gram negative bacteria have:
- plasma membrane
- periplasmic space
- thin peptidoglycan
- periplasmic space
- outer membrane (thick lipopolysaccharide and protein)
gram positive appears
purple after gram staining - remember purple happy face
gram negative appears
pink after gram staining - remember pink sad face
Capsule
Only some bacterium have it
4 steps in Gram Staining developed in 1884:
application of crystal violet to slide (purple dye)
Application of Iodine (mordant)
Alcohol wash (decolourisation)
Application of safranin (counter stain)
Gram staining, in addition ton being quick, simple and inexpensive is good because:
- Provides an early indication of the genus of bacteria that may
be causing infection. - Different species have varying capacity to cause disease
(pathogenicity). - Different classes of antibiotics are effective against Gram
positive and Gram negative bacteria -> targeted treatment
Aerobic Gram Positive cocci
- Staphylococci (clusters under microscope)
- Streptococci and Enterococci (chains under microscope)
What information can you get from the Gram Stain?
Whether the bacteria is a gram negative or gram positive bacteria and you can then go on to identify what strain it is before diagnosis and maybe even know how to handle the infection whether it’s removing the tissue or giving antibiotics.
Staphylococci – coagulase positive
Staphylococcus aureus
Staphylococci – coagulase positive
Staphylococcus aureus
Staphylococci – coagulase negative
- S. epidermidis
- S. haemolyticus
- S. saprophyticus.
Staphylococcus aureus
- Commensal organism carried in nose, axilla, perineum.
- A major human pathogen!
- Wide range of disease from boils/abscesses and soft tissue infections to septicaemia (infection in blood) and osteomyelitis (infection of the bone)
- Commonly penicillin resistant due to production of penicillinase.
note: gram positive bacteria that is coagulase positive
By a different mechanism, some strains of Staphylococcus aureus are
methicillin resistant
- Methicillin resistant Staph aureus (MRSA) poses major problems for infection prevention and control in hospitals
note: gram positive bacteria that is coagulase positive
Coagulase-negative staphylococci and plastics
bacteria form biofilms and may be significant pathogens in the presence of foreign bodies/prostheses e.g. prosthetic heart valves, prosthetic hip joints, pacemaker wires etc or
immunocompromised e.g. neonates / BMT with central
venous catheters.
Streptococci a- haemolytic
(partial haemolysis)
-> turn blood agar green
Can form a capsule to protect itself from antigens and so doesn’t get detected when in the body.
note: gram positive bacteria that is coagulase negative
Streptococci b-haemolytic
(complete haemolysis)
-> turn blood agar clear
note: gram positive bacteria that is coagulase negative
Streptococci Non haemolytic
no difference
note: gram positive bacteria that is coagulase negative
Streptococci is cultured in
blood agar
note: gram positive bacteria that is coagulase negative
Streptococcus pneumoniae (pneumococcus)
- a-haemolytic streptococci
- causes pneumonia, meningitis, septicaemia
note: gram positive bacteria that is coagulase negative
Viridans streptococci
- a-haemolytic streptococci
- Many different species
- Normal oral flora
- Cause of infective endocarditis
(infection of heart valves)
note: gram positive bacteria that is coagulase negative
b-haemolytic streptococci can be further identified by
carbohydrate surface antigens.
note: gram positive bacteria that is coagulase negative
b-haemolytic streptococci groups include:
- A (most serious)
- B (important)
- C
- D (important)
- F (important)
- G
Note: No E group
note: gram positive bacteria that is coagulase negative
Group A streptococci
Streptococcus pyogenes
- Major pathogen
- Pharyngitis, cellulitis, necrotising fasciitis (“flesh
eating bug”)
- threatening soft tissue infection
- need to remove infected tissue by surgery to get rid of
it.
note: gram positive bacteria that is coagulase negative
Group B streptococci
Streptococcus agalactiae
- Neonatal sepsis
- Meningitis
- Bacteraemia
- Genital tract carriage common (around 25% women).
- Also responsible for invasive infections in adults (often older and underlying factors e.g. diabetes, liver disease / alcohol abuse, CVD, malignancy).
note: gram positive bacteria that is coagulase negative
pregnant women and Streptococcus agalactiae
Pregnant women found to have this are given an antibody before birth to stop the bacteria being passed on to babies as they have an underdeveloped immune system and could contract meningitis if prevention methods aren’t taken.
They aren’t given the antibiotics months before as the bacteria could come back at the time of birth and infect the baby.
note: gram positive bacteria that is coagulase negative
Group D streptococci
Now re-classified as Enterococcus spp.
E.g. Enterococcus faecalis, Enterococcus faecium
- Often non-haemolytic.
- Found in the gut as a normal commensal.
- Cause of urinary tract infection and infective endocarditis
(infection of heart valves).
note: gram positive bacteria that is coagulase negative
Gram positive bacilli can be
- Aerobic
- Anaerobic (grow only in absence of oxygen)
Gram positive bacilli example
Clostridium difficile - “Difficult” to culture and major cause of diarrhea Clostridium perfringens Clostridium tetani - cause tetanus.
Asymptomatic gut carriage of Clostridium difficile in healthy people
up to 16% of adults and 66% of babies.