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.
Clostridium difficile
- Important cause of diarrhoea, associated with toxin production and potentially fatal.
- Increased risk of getting it with antibiotic use and anything else that disrupts the normal gut flora.
- Pseudomembranous colitis (inflammation of large intestine)
- Spread / transmitted via spores
- Detect antigen & toxin in stool sample by ELISA
bacterial spores are dangerous as they are
dormant structures (i.e. no metabolic activity) formed in response to adverse environmental conditions. They help in the survival of the organisms during adverse environmental conditions.
what is the best method for prevention of clostridium difficile
antibacterial gel won’t kill the bacteria but washing hands with soap is the best way to kill all types of bacteria not just this one.
especially after handling a patient who has had diarrhoea you should wash hands as this will prevent it from being passed onto someone else or yourself.
This bacteria produces spores so its extra important as it means that this bacteria can go into “hibernation” until its environment is more suitable.
Clostridium perfringens
- Found in soil and normal commensal in human and animal
gut/faeces. - Can contaminate food and cause gastroenteritis (enterotoxin producing strains).
- Infects wounds may can cause “gas gangrene”.
- example of a food borne infection
- also spread by spores
Clostridium tetani
- Toxin production by C. tetani cause tetanus.
- Uncontrolled muscle spasm due to loss of inhibition at neuromuscular junction.
- Antigenically modified toxin (toxoid) used for immunisation.
Clostridium botulinum
Source of “botox”
Bacillus anthracis
Cause of anthrax
Gram Negative bacteria:
Coliforms and other gram negative bacilli including;
- Neisseria spp
- Moraxella spp
- Haemophilus influenzae
- Campylobacter spp
- Helicobacter spp
- Bacteroides
- other anaerobes
Mainly commensals of the human large intestine.
Miscellaneous bacteria:
Those not readily classified by Gram stain, including:
- Mycobacteria
- Spirochaetes
- Chlamydia
- Mycoplasma.
Colonisation means
germs are on the body but do not make you sick. People who are colonised will have no signs or symptoms.
Infection means
that germs are in or on the body and make you sick, which results in signs and symptoms such as fever, pus from a wound, a high white blood cell count, diarrhoea, or pneumonia.
75% of “terrible twelve” are
Gram Negative bacteria!
Gram negative cocci
- Neisseria spp
- Moraxella catarrhalis
note: gram negative bacteria
the glass pressed against skin test is used for
One sign of meningococcal septicaemia is that the rash doesn’t fade when you apply pressure to the skin.
If you ca see the spots clearly through the glass, this can be a sign of septicaemia (blood poisoning) and can lead to meningitis.
one strain of Neisseria spp is
Neisseria meningitidis (meningococcus).
- Meningitis – inflammation of the meninges and septicaemia.
- Important when isolated from a sterile site e.g. blood cultures
or CSF.
- PCR available on EDTA blood for N. menigitidis.
- Often life threatening
note: gram negative bacteria
another strain of Neisseria spp is
Neisseria gonorrhoeae
- Causes urethritis in men and pelvic inflammatory disease in
women.
- Spread by sexual contact.
note: gram negative bacteria
Moraxella catarrhalis
- Causative agent of respiratory tract infections.
- Especially in those with underlying lung pathology
note: gram negative bacteria
Coliforms are
organisms which are in the family Enterobacteriaceae
Lactose fermentation is a useful preliminary test in classifying
Gram negative bacilli
Escherichia coli
- Human and animal reservoirs.
- Several virulence mechanisms: pili, capsule, endotoxin and exotoxins.
- Ferments lactose.
- Important cause of urinary tract infection (UTI)
- Over 160 serotypes based on O antigen (LPS).
- Strains vary considerably in disease potential.
note: gram negative bacteria
Escherichia coli and diarrhoea
Enterotoxogenic E.coli
- commonest cause of traveller’s diarrhoea
Enterohaemorrhagic E.coli
- bloody diarrhoea
- haemolytic uraemic syndrome (HUS) associated with E.coli O157
note: gram negative bacteria
Salmonella spp.
- Does not ferment lactose.
- Second commonest cause of bacterial diarrhoea in UK.
- Can be invasive i.e. enters bloodstream.
- Salmonella enterica >1500 serotypes.
- Whole genome sequencing now performed in reference lab.
- Self-limiting enterocolitis with or without bloody diarrhoea (incubation period 12-48 hrs).
note: gram negative bacteria
Salmonella typhi
Distinct from Salmonella enterica:
- Cause of typhoid fever.
- Fever, constipation early stages.
- Organism can be isolated from blood cultures as well as
faeces.
- Risk during foreign travel but vaccine available
note: gram negative bacteria
Other coliforms include
Shigella spp
– Cause of diarrhoea, dysentery – in UK usually
travel related.
Klebsiella spp
– Cause of UTI and hospital acquired pneumonia.
Proteus spp
– Causes UTI – often associated with stones.
note: gram negative bacteria
Curved Gram Negative bacilli
Campylobacter spp.
- Commonest cause of bacterial diarrhoea in UK.
- Microaerophilic – likes low O2 . content
- Source is domestic animals and chickens.
- Spread via faecal-oral route.
- Foul smelling -> bloody diarrhoea (incubation period 2-5
days) .
Helicobacter pylori
- Curved rods.
- Natural habitat is human stomach.
- Damages mucosa and causes ulcers.
- Strong risk factor for gastric adenocarcinoma
note: gram negative bacteria
Haemophilus influenzae
- Cocco-bacilli (mixed appearance).
- Causes respiratory tract infection (second to S. pneumoniae).
- Capsulate form (type “b”) was formerly an important cause of meningitis in children (Hib conjugate vaccine now given).
note: gram negative bacteria
Pseudomonas spp
- Water and soil coloniser – drains, sinks, mops.
- May contaminate medical equipment.
- Hospital acquired cause of sepsis: e.g. UTI, bacteraemia, pneumonia (rare).
- Feared respiratory pathogen in cystic fibrosis.
- Multi-drug resistance mechanisms.
- Limited treatment options.
note: gram negative bacteria
Anaerobes - gram negative bacteria
Often part of a polymicrobial infection. Increasing importance and recognition. - Bacteriodes spp. - Prevotella - Porphyromonas
Bacteroides fragilis
- Part of normal colonic flora.
- Causes intra-abdominal abscess.
- May spread to other sites
note: gram negative bacteria
Oral Anaerobes include
- Prevotella
- Porphyromonas e.g. P. gingivalis
- Pasteurella spp
- Capnocytophyga spp
- Important in aetiology of periodontal disease and may be part of polymicrobial dental abscesses.
- Role in aspiration pneumonia.
- Human & animal bite infections
note: gram negative bacteria
Miscellaneous bacteria
Not all bacteria can be stained with Gram’s method.
Not all bacteria can be cultured by standard methods:
– Mycobacterium spp.
– Spirochaetes.
– Chlamydia / Chlamydophila.
Acid And Alcohol Fast Bacilli (AAFB)
- Resistant to decolourisation by acid or alcohol after
staining with carbol fuchsin. - Mycobacterium species are visualised with special
stains e.g Ziehl-Neelsen (ZN) or Auramine
Laboratory identification of Mycobacteria
- Does not identify species!!!
- ZN or Auramine phenol (screening).
- Culture required for ID.
- New automated liquid culture.
- Growth in 2-4 weeks.
- Molecular detection and identification of MTB possible.
Tuberculosis
- Caused by Mycobacterium tuberculosis.
- Link with HIV.
9 million new cases worldwide
– India accounted for 24% of new diagnoses.
– 3.5% of these new cases were Multi-Drug
Resistant (MDR-TB).
1.1 million/9 million HIV co-infected
– 1.5 million deaths.
Atypical mycobacteria can cause
disease.
there is a wide range of environmental mycobacteria
Leprosy
Mycobacterium leprae – cannot be cultured.
- Mainly India, Nepal, Brazil, parts of Africa.
- Around 250,000 new cases per year worldwide.
- spread through infected droplets
- Attacks peripheral nerves.
Spirochaetes
- Long, spiral-shaped bacteria.
- Not easily visualised by light microscopy.
- Very difficult to culture (never Treponema pallidum).
- Dark ground microscopy or immunofluorescence.
- Often diagnosed by serology.
Common spirochaete diseases
Organism Disease
Treponema pallidum Syphilis
Borrelia burgdorferi Lyme disease
Leptospira interrogans Leptospirosis
Syphilis and stages
- 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
Diagnosis – serology, nucleic acid amplification tests
(NAAT).
• Part of antenatal screening.
• Management – antibiotics, contact tracing and
screening.
Lyme Disease and stages
Transmitted by ticks.
- Stage 1: Skin rash (erythema chronicum migrans) 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
- Diagnosed by clinical assessment and serology (antibody detection)
Leptospirosisand stages
Infects animals and concentrates in the kidneys.
- Spread is via infected urine and other body fluids and
tissues (zoonosis).
- At-risk groups
– sewage workers
– watersports.
- Range of clinical presentations
– Weil’s disease – febrile illness with systemic upset, liver
and renal failure, aseptic meningitis, 10 % mortality
Chlamydia
- Obligate intracellular bacteria.
- Culture only in cell lines (do not grow on agar).
- Can be diagnosed by serology.
- Respiratory Infection:
– Chlamydophila pneumoniae
– Chlamydophila psittaci (psittacosis) - contact with birds - Ophthalmic and genital tract infection
Chlamydia trachomatis
- Commonest Sexually Transmitted Infection (STI) in the UK.
- Often asymptomatic in women, can lead to pelvic inflammatory disease and infertility.
- Trachoma (tropical eye infections)
- Genital and neonatal infection
- Neonatal infections
Diagnosis of Chlamydia trachomatis
– nucleic acid amplification tests (NAAT) on first void urine
or vulval/vaginal swabs.
Cocci
round
Bacilli
rod-shaped
Peptidoglycan/murein gives cell
strength and shape
Lipopolysaccharides(LPS), elicit strongimmune responsesin animals and humans.
Flagellum types
A-Monotrichous Vibrio cholerae (1);
B-Lophotrichous Spirillum spp (One point);
C-Amphitrichous Rhodospirillum rubrum (both sides);
D-Peritrichous (all over)
- used for motility
Fimbriae used for
adherence
Other bacterial features include
Spores
conjugation is a mechanism of
gene transfer that requires direct contact between donor and recipient cells.
F+ is donor, F- is recipient
step 1: contact between donor and recipient
step 2: plasmid becomes mobilised and endonuclease cleaves plasmid at origin of transfer
step 3: plasmid id synthesised via the rolling circle mechanism
step 4: once inside the other cell, complementary dna is synthesised
eventually all cells become F+
