classification of pathogenic bacteria, pathogens & 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)
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 purple2 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 areasCause of infective endocarditis
beta haemolytic streptococci
(complete haemolysis)Turn blood agar clearFurther identified by carbohydrate surface antigens: groups A-G), A,B,D,F clinically most important A. streptococcus pyogenesb. streptococcus agalactiaeD. 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 commoninvasive 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 gonorrhoeaeMoraxella catarrhalis
Neisseria meningitidis
gram -ve coccus meningitis and septicaemiaimportant 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 reservoirs160 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 enterohaemorrhagicDon’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 stagesfatal if untreated- Organism can be isolated from blood cultures as well as faecesRisk during foreign travel but vaccine available
curved gram -ve bacilli
Campylobacter sppHelicobacter pylorivibrio 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 methodsMycobacterium 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 wksMolecules 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 lesion2y - generalised systemic illness and rash, latent phase - symptomatic episodes may occur3y -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 infectionChlamydia 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 variantsleads 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 leukaemiasHuman 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/artificialantigen stimulates immune responselong 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 hostusually promote a full, long-lasting antibody response after one or two dosesreversion to wild-type organism may occur contraindicated in some individualsrequire 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 requiredsome 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 formalintoxin 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 defencesphagocytic cells
acquired immunity
specific response to antigen leads to immunological memory humoral and cellular
phagocytosis
polymorphs (neutrophils, eosinophils, basophils) and monocytes in bloodmononuclear phagocytic system: spleen, liver and regional lymph nodesother white cells
antibody and complement
involves immunoglobulinsneutralised 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 infectionmacrophages 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 dosefaecal-oral spreadbacillary dysentery some strains produce toxin4 species
