Bacterial Pathogens Flashcards
How do bacteria tend to be classified?
By their Gram stain and then their shape
e.g. Gram negative bacillus
What is the difference in cell structure between a Gram + and Gram - bacterial cell?
Gram positive:
- thick cell wall that retains Gram stain
- more resistant to drying so is often found on the skin
Gram negative:
- thinner cell wall that does not retain gram stain
- they have a fatty outer cell membrane made from lipopolysaccharides
- requires moisture to survive
- often found in the large bowel and moist areas of the skin
What is the difference between a coccus and a bacillus?
Cocci are more circular in shape
Bacilli are rod-shaped
What are the 3 intervening factors that explain why certain bacteria cause particular infections?
Host factors:
- devices
- immune system
Opportunity:
- normal flora
- exposure
Bacterial factors:
- virulence
- resistance
- environmental survival
What is meant by “devices” as a host factor determining why bacteria cause particular infections?
Devices act as a portal of entry for infection
e.g. Catheters and lines when in hospital
What is meant by “immune system” as a host factor?
infections are more common in immunocompromised patients
e.g. Leukaemia, patients on immunosuppressants after transplants
What is meant by “virulence” as a bacterial factor?
Virulence factors are different in all bacteria
they allow for enzyme production, breakdown of tissues and evasion of host immune defences
Depending on the bacterium, they wil have particular virulence factors which may make them more able to cause infection
What is meant by “environmental survival” as a bacterial factor?
some bacteria are more resistant to environmental stresses
e.g. Drying out, formation of biofilms allows them to adhere to things such as plastic
How are “normal flora” involved in opportunity?
the proximity of the normal flora to the site of infection has an influence
e.g. Anal bacteria getting into a place that they shouldn’t after colonoscopy
What is the most common bacteria to cause UTIs?
In which group is this more prevalent and why?
E. Coli
Females are more prone to UTIs as they have a shorter urethra
This is a shorter distance for the bacteria to ascend
Which areas are colonised in a UTI caused by E. Coli?
they colonise the urethral meatus and the surrounding area
they are able to adhere to uroepithelal cells and urinary catheter materials
What is significant about UTI by E. Coli?
How do they cause symptoms?
They trigger the host inflammatory response, leading to pain and dysuria
They are able to develop resistance to antibiotics
What is the most common bacterium to cause skin infections?
Where is it usually found?
Stapyloccoccus aureus
It is found in the nasal carriage of 50% of healthy people
It is able to adhere to damaged skin (e.g. burns, skin conditions, abrasion)
What is the mechanism of action behind an S. aureus skin infection?
it produces exoenzymes and toxins that can damage tissues
this provokes the host inflammatory response
e.g. pus formation
What is meant by Staphylococcus aureus being a primary pathogen?
it can cause infection in normal healthy people
(not immunocompromised)
What types of infection tend to be caused by Staphylococcus aureus?
how is this organism classified?
Gram positive coccus
It causes skin and soft tissue infections, including surgical site infections
In severe cases it can cause bacteriaemia and septicaemia
What are less common conditions caused by Staphylococcus aureus?
- osteomyelitis
- septic arthritis
even less common:
- endocarditis
- pneumonia
- UTI
- meningitis
What is meant by bacteraemia?
Bacteriaemia is the presence of bacteria within the blood stream
What does Staphylococcus aureus look like under the microscope?
Gram positive coccus
”Staphylo” means bunch of grapes
What type of bacteria is staphylococcus epidermidis?
Where does it tend to be found?
Coagulate negative staphylococci
it is an opportunistic pathogen and is carried on the skin of most people
How does staphylococcus epidermidis tend to cause infection?
It causes infection in association with foreign bodies (e.g. catheters, prosthetic joints)
it adheres to plastics/metals using glycocalyx, forming biofilms
(this is in the period where the prosthetic joint is exposed to air, before surgical insertion)
this leads to loosening of the prosthetic joint and instability and pain in the joint
What is an alternative name for Streptococcus pyogenes?
What is it the commonest cause of?
Group A Strep
It is a Gram positive coccus
It is the commonest cause of bacterial sore throat
What other conditions are caused by streptococcus pyogenes?
- Scarlet fever
- Necrotising fasciitis
- Invasive infections, such as pneumonia
- Puerperal sepsis
- Secondary immunological presentations, such as glomerulonephritis
What is a typical feature of scarlet fever?
“Strawberry tongue”
What is meant by a “secondary immunological presentation” in streptococcus pyogenes?
antibodies to the group a strep can attack human tissue
this leads to an autoimmune condition
What does streptococcus pyogenes look like under the microscope?
“Strepto” is Ancient Greek for “chain”
What conditions are caused by streptococcus pneumoniae?
- Commonest cause of bacterial pneumonia
- Commonest cause of bacterial meningitis (except in neonates)
- Can cause other common childhood infections, such as otitis media
What is an alternative name for Streptococcus agalactiae?
What does it cause?
Group B Strep
it is the commonest cause of bacterial meningitis and sepsis in NEONATES
(babies under three months)
What is meant by ‘Streptococcus Milleri complex’?
What conditions are they associated with?
Three closely related species of pus-forming streptococci
They are associated with abscesses - dental, lung, liver, brain and intra-abdominal
What is meant by “viridans streptococci”?
What do they typically cause?
Collective name for a number of species of a-haemolytic streptococci that inhabit the upper respiratory tract
they are a classic cause of sub-acute bacterial endocarditis
How do viridans streptococci cause sub-acute bacterial endocarditis?
bacteraemia results from brushing teeth, biting lip etc. and bacteria entering the bloodstream
if a patient has a disease of the heart valve (e.g. aortic stenosis), altered blood flow causes the organisms to adhere to the valves
What is streptococcus gallolyticus?
What is it associated with?
it is a type of a-haemolytic streptococcus that forms part of normal bowel flora
bacteraemia with this organism can be associated with colonic malignancies
What type of bacteria is Listeria monocytogenes?
What does it cause?
Gram positive bacillus
it is a rare (but significant) cause of sepsis and meningitis in pregnancy, neonates and immunosuppresed patients
What are the significant features of listeria monocytogenes?
What is it associated with the consumption of?
Zoonosis:
- it is of animal origin
- it is able to grow at low temperatures
It is associated with consuming cheese made from unpasteurised milk and other foodstuffs
What type of bacteria are the Corynebacterium species?
Where are they usually found and how do they cause infection?
Gram positive bacilli that are usually not pathogenic
A number of species are commensals of the skin and upper respiratory tract
occasional opportunistic infections are associated with devices and trauma
What is the most well-known member of the Corynebacterium species and what does it cause?
Corynebacterium diphtheriae
It causes diphtheria, but is rarely seen in the UK due to the immunisation
What type of bacteria is Propionibacterium acnes?
What does it cause?
Gram-positive bacillus
It is referred to as Cultibacterium acnes and is associated with acne
It can also cause device-associated and post-procedural infections
What is meant by Enterobacteriaceae (‘coliforms’)?
What are typical common species?
a collective term for a number of species of gram negative bacilli found in bowel flora
Common species include:
- Escherichia coli
- Klebsiella pneumoniae
- Enterobacter cloacae
What is E. Coli the commonest cause of?
- Urinary tract infections
- Bacteriaemia - sources include urinary, biliary and intra-abdominal
What other type of infections are caused by E. Coli?
Nosocomial infections:
- these include line infections, pneumonia and wound infections (post-surgery)
Toxigenic strains:
- Associated with severe diarrhoea and haemolytic uraemic syndrome
- this can lead to renal failure
What type of bacteria is pseudomonas aeruginosa?
How can it be identified?
Multi-resistant gram negative bacillus
It is an opportunistic pathogen that produces a characteristic green pigment
What does pseudomonas aeruginosa typically cause?
- Respiratory infections (in underlying chronic lung disease)
- UTIs
- Soft tissue infections (post bone marrow transplant)
In vulnerable patients
What type of bacteria is neisseria meningitidis?
What does it cause?
Gram negative diplococcus
It causes meningococcal sepsis and/or meningitis
What is the classic presentation of an infection with neisseria meningitidis?
a purpuric non-blanching rash
this is characteristic of sepsis
What type of bacteria is Neisseria gonorrhoeae?
What does it cause?
Gram-negative diplococci
it causes gonorrhoea (STI)
it causes ophthalmia neonatorum when a child is born to a mother with active gonorrhoea
it can rarely cause invasive infections (e.g. septic arthritis) secondary to the primary STI
What type of bacteria is Haemophilus influenzae?
Where is it found and what does it tend to cause?
Gram negative bacillus
it forms part of the normal respiratory tract flora
It causes respiratory tract infections such as pneumonia, infective exacerbations of COPD
What is the different type of Haemophilus influenzae?
capsulated types (e.g. type b)
these are associated with meningitis and epiglottis
only type b infections are prevented by the HIb vaccine
What is meant by an anaerobe?
a bacterium which grows in the absence of oxygen
What are the 4 main anaerobes and what do they cause?
Clostridium species:
- many are spore-forming organisms
C. Difficile:
- antibiotic-associated diarrhoea/colitis
C. Perfringens:
- classical cause of gas gangrene
C. Tetani:
- cause of tetanus
C. botulinum:
- cause of botulism (food poisoning that results in paralysis)
What types of infections do anaerobes tend to be part of?
polybacterial infections
e.g. Dental infections, lung abscesses, colonic abscesses, post-trauma skin/soft tissue infections
What are Mycobacterium species often referred to as and why?
Acid Fast Bacilli (AFBs)
They do not stain using convention Gram staining, acid fast stain is needed
What is the most well known Mycobacterium species and what does it cause?
Mycobacterium tuberculosis
this causes TB
What may other mycobacterium species be referred to as?
What do they cause?
atypical mycobacteria
they cause respiratory infections in those with chronic lung disease
or
opportunistic infections in immunocompromised patients
What are the 2 main bacteria without a conventional cell wall?
Chlamydia species:
- C. Trachomatis is the commonest cause of STI
Mycoplasma species:
- M. Pneumoniae is the commonest cause of respiratory tract infections
What are spirochaetes?
What do they cause?
they are spiral shaped bacteria that do not stain with Gram stain
Treponema pallidum:
- this causes syphylis
Other species of spirochaetes can cause leptospirosis and Lyme disease
