classification of pathogenic bacteria, pathogens and host Flashcards
gram’s stain
- Quick, simple, inexpensive way to classify clinically important organisms
- Provides an early indication of the genus of bacteria that may be causing infection
○ Combine knowledge from gram stain with morphology - Different species have varying capacity to cause disease (pathogenicity)
Different classes of antibiotics are effective against gram +ve and -ve bacteria (targeted treatment)
- Provides an early indication of the genus of bacteria that may be causing infection
cells wall differences in gram +ve vs -ve bacteria
Thick peptidoglycan layer in gram +ve
Thin peptidoglycan layer, more complex cell wall, thick lipopolysaccharide layer in gram -ve
classification of gram +ve bacteria
stain purple
2 major types of classification are aerobic/anaerobic or their morphology (cocci/bacilli)
Chains - streptococcus
Clusters - staphylococci
Streptococcus - how do they grow on blood agar
Partial/complete/no haemolysis
coagulase +ve gram +ve bacteria
staphylococcus aureus
staphylococcus aureus
○ Commensal organism carried in nose, axilla and perineum
○ Major human pathogen causing a wide range of disease
boils/abscesses and soft tissue infections to septicaemia and osteomyelitis
- Commonly penicillin resistant due to production of penicillinase
- By a different mechanism, some strains are methicillin resistant
MRSA poses major problems for infection prevention and control in hospitals
coagulase -ve gram +ve bacteria
- many different species (staphylococcus
S. epidermis, S. haemolyticus, S. saprophyticus., S. lugdunensis - has many of the virulence factors of S. aureus) - Mainly skin commensals
- can be pathogenic in the presence of foreign bodies or immunocompromised
alpha haemolytic streptococci
partial haemolysis - Turn blood agar green
Streptococcus pneumoniae (pneumococcus)
Viridans streptococci
Streptococcus pneumoniae
alpha haemolytic
pneumonia, meningitis, septicaemia
Produces capsule to protect S. pneumoniae from the host so it cant be recognised as foreign
Viridans streptococci
○alpha haemolytic
○ Normal oral flora, cause infection in other areas
Cause of infective endocarditis
beta haemolytic streptococci
(complete haemolysis)
Turn blood agar clear
Further identified by carbohydrate surface antigens: groups A-G),
A,B,D,F clinically most important
A. streptococcus pyogenes
b. streptococcus agalactiae
D. Enterocossu faecalis, Enterococcus faecium
streptococcus pyogenes
group A beta haemolytic
major pathogen -
pharyngitis, cellulitis, necrotising fasciitis
(skin and soft tissue infections, bone and joint infections and can lyse human muscle and connective tissue, can lead to sepsis)
streptococcus agalactiae,
group B beta haemolytic neonatal sepsis (meningitis, bacteraemia), genital tract carriage common
invasive infections in adults often older and underlying factors e.g. diabetes, liver disease/alcohol abuse, CVD, malignancy )
enterococcus
group D beta haemolytic
often non-haemolytic,
enterococcus faecalis
found in the gut as normal commensal, cause of UTI and infective endocarditis
classification of gram +ve bacilli
- Aerobic
Anaerobic (grow only in the absence of oxygen)
spore forming/non-spore forming
classification of gram +ve cocci
coagulase test
blood agar culture - haemolysis
CLOSTRIDIUM DIFFICILE
gram +ve bacilli
- Asymptomatic gut carriage in healthy people
- Important cause of diarrhoea, associated with toxin production and potentially fatal (toxins found in stool sample)
- Increased risk with antibiotic use and anything else that disrupts the normal gut flora
- Pseudomembranous colitis - inflammation of the large intestine
- Spread/transmitted via spores
CLOSTRIDIUM PERFRINGENS
- gram +ve bacilli
Found in soil and normal commensal in human and animal gut/faeces - Spread by spores
- Can contaminate food and cause gastroenteritis (enterotoxin producing strains)
Infects wounds and can cause gas gangrene
CLOSTRIDIUM TETANI
- gram +ve bacilli
- Spread by spores
- Toxin produced by C. tetani can produce tetanus
○ Uncontrolled muscle spasm
Antigenically modified toxin (toxoid) used for immunisation
colonisation
bacteria grow on body sites exposed to the environment, without causing any infection. This is a normal process.
infection
presence of microorganisms causing damage to body tissues, usually in the presence of acute inflammation
gram -ve cocci
Neisseria meningitidis
Neisseria gonorrhoeae
Moraxella catarrhalis
Neisseria meningitidis
gram -ve coccus
meningitis and septicaemia
important when isolated from a sterile site (e.g. blood cultures or CSF, gram staining is very important to identify the bacteria here), PCR available on EDTA blood for N. meningitidis, often life threatening
Neisseria gonorrhoeae
gram -ve coccus
causes urethritis in men and pelvic inflammatory disease in women, spread by sexual contact, causes gonorrhoea, identical appearance to Neisseria meningitidis on gram stain (differentiate by clinical findings)
Moraxella catarrhalis
gram -ve coccus
causative agent of respiratory tract infections, especially in those with underlying lung pathology
what are coliforms
organisma which are in the family Enterobacteriaceae
• Mainly commensals of the human large intestine (gut commensals)
• Key example is E. coli
Lactose fermentation is a useful preliminary test in classifying gram -ve bacilli (can use selective agar)
ESCHERICHIA COLI
-gram -ve bacilli
Human and animal reservoirs
160 serotypes, strains vary in terms of disease potential
- Sexual virulence mechanisms: pili, capsule endotoxin and exotoxins
- Major form of food borne infection
- Ferments lactose
- Important cause of UTI and septicaemia, also neonatal sepsis
- enterotoxogenic and enterohaemorrhagic
Don’t treat with antiobiotics - these could lead to more toxins being produced
SALMONELLA SPP.
-gram -ve bacilli
- Major form of food borne infection
- DOES NOT ferment lactose
- Salmonella enterica:
Self-limiting enterocolitis with or without bloody diarrhoea
- 2nd commonest cause of bacterial diarrhoae in the UK
- Can be invasive i.e. enters bloodstream
SALMONELLA TYPHI
- gram -ve bacilli
Distinct from Salmonella enterica - Cause of typhoid fever
- Fever, constipation early stages
fatal if untreated - Organism can be isolated from blood cultures as well as faeces
Risk during foreign travel but vaccine available
curved gram -ve bacilli
Campylobacter spp
Helicobacter pylori
vibrio spp
Campylobacter spp.
curved gram -ve bacilli
microaerophilic (likes low oxygen), source is domestic animals and chickens, spread via faecal-oral route, foul smelling –> bloody diarrhoea (incubation period 2-5 days), commonest cause of bacterial diarrhoea in UK , no vaccine currently, often seen in springtime, can enter the bloodstream, not treated with antibiotics unless very severe
Helicobacter pylori:
curved gram -ve bacilli
natural habitat is human stomach, damages mucosa and causes ulcers, strong risk factor for gastric adenocarcinoma (oncogenic organism), treat to avoid future cancer risk
Haemophilus influenzae
gram -ve cocco-bacilli (mixed appearance), causes respiratory tract infection (2y to S, pneumoniae), capsulate for (type b) was formerly an important cause of meningitis in children, other no-typable forms of H. influenzae still able to cause meningitis
Pseudomonas SPP - P. aeruginosa
anaerobic gram -ve bacilli- Water and soil coloniser - drains, sinks, mops - May contaminate medical equipment - Can colonise upper resp tract - Hospital acquired cause of sepsis e.g. UTI, bacteraemia, pneumonia (rare) - Feared respiratory pathogen in CF - Multi drug resistance mechanisms Limited treatment options
anaerobes
- Often part of polymicrobial infection e.g. liver/brain abscess
- Increasing importance and recognition
- Bacteroides spp, Prevotella, Porphyromonas
- Difficult to grow in the lab due to their sensitivity to oxygen
Bacteroides fragilis
• Part of normal colonic flora
• Causes intra-abdominal abscess
• May spread to other sites e.g. blood
oral anaerobes
- 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 and animal bit infections
miscellaneous bacteria
- Not all bacteria can be stained w/ Gram’s method
- Not all bacteria can be cultured by standard methods
Mycobacterium spp, spirochaetes, chlamydia/chlamydophila
acid and alcohol fast bacteria
• Resistant to decolourisation by acid or alcohol after staining with carbol fuchsin
Mycobacterium species are visualised with special stains e.g. ZN or auramine
lab identification of mycobacteria
• ZN or auramine phenol screening • Doesn’t identify species • Culture required for ID ○ New automated liquid culture ○ Growth in 2-4 wks Molecules detection and identification of MTB possible
tuberculosis
• Caused by mycobacterium tuberculosis
• Linked with HIV
• Recommended rapid diagnostic nucleic acid amplification tests for diagnosing pulmonary (including laryngeal) TB in adults if there is:
Clinical suspicion of TB disease and the person has HIV
In circumstances in which rapid info about mycobacterial species would alter the person’s care
In a situation where a large contact tracing initiative is being exploited
Mycobacterium leprae
cant be cultured, mainly India, Nepal, Brazil, parts of Africa, attacks peripheral nerves
SPIROCHAETES
mycobacteria
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: Treponema pallidum (syphilis), Borrelia burgdoferi (lyme disease), Leptospira interrogans (Leptospirosis)
SYPHILIS
spirochaete
treponema pallidum
1y - non-painful skin lesion
2y - generalised systemic illness and rash, latent phase - symptomatic episodes may occur
3y -CNS, 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
mycobacteria
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, wks/mths later when patients suffer cardiac or neurological and muscoloskeletal symptoms, stage 3: chronic disease, occuring yrs later when patients present with chronic skin, nervous system or joint abnormalities. Diagnosed by clinical assessment and serology (antibody detection)
leptospirosis
mycobacteria
infects animals and concentrates in the kidneys, spread is via urine and other body fluids and tissues (zoonosis), at risk groups (sewage workers, water sports), range of clinical presentations (Weil’s disease: febrile illness with systemic upset, liver and renal failure, aseptic meningitis, 10% mortality)
chlamydia
obligate intracellular bacteria, cultured only in cell lines, can be diagnosed by serology, respiratory infection (chlamydophila pneumoniae, Chlamydophila psittaci (psittacosis) - contact with birds), ophtalmic and genital tract infection
Chlamydia trachomatis: trachoma (tropical eye infections), genital and neonatal infection, diagosis: NAAT on 1st void urine or vulval/vaginal swabs, often asymptomatic in women, can lead to pelvic inflammatory disease and infertility
define commensal
organism which is part of the normal flora e.g. E. coli in the gut, S. aureus in the nose
define pathogen
organism which can cause disease
what are the requirements for a micro-organism to cause infection
infectivity and virulence
infectivity
ability to become established on or within a host
○ Attachment: E. coli (P-fimbriae, receptor on uroepithelial cells)
Acid resistance: Helicobacter pylori - lives in the stomach and causes gastritis and gastric ulcers (urease, makes ammonia from urea)
virulence
capacity to cause harmful effects (disease) once established
○ Conferred by virulence factors: genetically determined microbial components
§ Invasiveness, toxin production, evasion of immune system
□ INVASIVENESS: Streptococcus pyogenes (Group A streptococci), necrotising fasciitis, cellulitis, connective tissue breakdown (hyaluronidase, collagenase), fibrinolysis (streptokinase)
Specific to strains, not species
exotoxins
released extracellularly by the microorganism
enterotoxins
exotoxins which act on the GI tract
example of an exotoxin
e.g. SUPERANTIGENS: Certain exotoxins of S. pyogenes and S. aureus. Able to stimulate division of T cells in the absence of specific antigen. Overwhelming cytokine production causes toxic shock
examples of enterotoxins
CHOLERA: Vibrio cholera, colonises S intestine, enterotoxin production (increases cAMP levels, inhibits uptake of Na+ and Cl- ions, stimulates secretion of Cl- and HCO3- ions. Passive (massive) outflow of water, causes death by dehydration, treated by rehydration
endotoxins
structurally part of the gram -ve cell wall
examples of endotoxins
Lipopolysaccharide (Lipid A, oligosaccharide core, specific polysaccharide chain), E. coli and other gram -ve bacilli, Neisseria meningitidis, induces severe uncontrolled host response (cytokine production, fever, rigors, hypotension, tachycardia, collapse
superantigens
- Certain exotoxins of S. pyogenes and S. aureus
- Able to stimulate division of T cells in the absence of specific antigen
- Overwhelming cytokine production causes toxic shock
influenza A
- Virus infects cells of the respiratory tract
- Destruction of respiratory epithelium which can lead to secondary bacterial infections
- Altered cytokine expression leading to fever
• Non-human host for influenza a play a key role in generating new virus types through antigenic shift
antigenic drift in influenza A
minor changes, in the genes of flu viruses, occurs gradually over time to generate antigenic variants
leads to a generation of novel influenza viruses
antigenic shift in influenza A
(abrupt major changes in antigenic structure)
leads to generation of novel influenza viruses
enterovirus infections (5)
- Poliomyelitis (poliovirus)
- Aseptic meningitis (many enteroviruses)
- Myocarditis (coxsackie B viruses)
- Pancreatitis (coxsackie B viruses)
Respiratory infections (many enteroviruses)
example of a latent virus infection
herpes simplex virus - cold sores (type 1) and genital lesions (type 2)
virus induced tumours
- E.g. papillomaviruses - cervical carcinoma, Retroviruses - lymphomas and leukaemias
Human T - lymphotropic virus 1 (HTLV-1): transmission by blood and mother to child, infects T cells, modifies host cell gene expression using a transactivating protein, variety of diseases including leukaemia (adult T cell lymphoma-leukaemia), directly responsible for the tumour
active immunisation
natural/artificial antigen stimulates immune response long term immunity immunological memory no immediate effect but faster and better response to next antigenic encounter
live attenuated vaccine
attenuation of a pathogenic organism by repeated passage in cell culture or non-human host
usually promote a full, long-lasting antibody response after one or two doses
reversion to wild-type organism may occur
contraindicated in some individuals
require refrigeration until administration
killed vaccine
inactivated pathogenic organism
reversion to wild type organism doesnt occur
promote weaker immune responses in comparison to live vaccines - multiple doses may be required
some inactivated vaccines contain adjuvants
possible side effects of producing inflammatory responses
adjuvants
substances that enhance the antibody response
examples of attenuated vaccine
MMR, BCG, polio, varicella zosterm, yellow fever
examples of killed vaccines
polio, hep A, rabies
toxoid vaccine
toxin treated with formalin
toxin retains antigenicity but has no toxic activity
only induces immunity against the toxin and not the organism that produced it
examples of toxoid vaccine
tetanus, diphtheria
define latent infection
type of infection that may occur after an acute episode; the organism is present but symptoms are not; after time the disease can reappear
define asymptomatic infection
patient is a carrier for a disease or infection but experiences no symptoms
host defence mechanisms
innate and acquired immunity
phagocytosis
antibodies and complement
cell mediated immunity
innate immunity
physical defences
phagocytic cells
acquired immunity
specific response to antigen
leads to immunological memory
humoral and cellular
phagocytosis
polymorphs (neutrophils, eosinophils, basophils)
and monocytes in blood
mononuclear phagocytic system: spleen, liver and regional lymph nodes
other white cells
antibody and complement
involves immunoglobulins
neutralised toxin, neutralised virus, prevents adherence of microorganisms, opsonises capsulate organisms, useful in diagnosis
humoral immunity: mostly bacterial infection, extra-cellular, acute inflammation, neutrophilia
cell mediated immunity
combat intracellular infection
macrophages present antigen and stimulate T cells, cytokines produced and control the response
enterotoxogeic E coli
commonest cause of traveller’s diarrhoea (contaminated water)
- Enterohaemorrhagic E. coli
more severe form, bloody diarrhoea, haemolytic uraemic syndrome (HUS) associated w/ E. coli O157
shigella
not lactose fermenting low infective dose faecal-oral spread bacillary dysentery some strains produce toxin 4 species
