Bacteria Flashcards
Which one of the following structures is NOT found in Gram-positive cell walls?
Lipopolysaccharide
MacConkey’s Agar plates are
A selective media for the growth of lactose fermenting gram negative bacteria
A positive result in which of the following bacteria diagnostic tests would generate oxygen?
Catalase
Which Lancefield Group does the bacterium Streptococcus pyogenes belong to?
Group A
What colour does Streptococcus pyogenes stain in a Gram stain?
Purple
What result will Streptococcus pyogenes give in a catalase test?
Negative
Streptococcus pyogenes growth on blood agar plates reveals…
Beta (clear) haemolysis
TSST-1 is a bacterial toxin that can cause systemic effects on the host leading to toxic shock syndrome. It is secreted by..
Staphylococcus aureus
Which of the following bacterial pathogens can evade the host immune response by secreting Protein A, which binds to the Fc portion of IgG molecules?
Staphylococcus aureus
A bacterial cell has a typical diameter of…
1 micrometre
The bacterial adhesins of Neisseria gonorrhoeae and Escherichia coli are in the form of..
Pili
A sputum sample is cultured, and a Gram-positive encapsulated bacterium is cultured from the sample that is catalase negative and Optichin sensitive. What is the most likely identity of this bacterium?
Streptococcus pneumoniae
Streptococcus pneumoniae bacterium’s polysaccharide capsule is important in the pathogenesis of this disease presentation because..
It prevents phagocytosis by macrophages and neutrophils
A 15-day old female infant (birth weight 2.6kg) presents with fever and respiratory failure. A cloudy cerebrospinal fluid sample from the patient showed 1042 cells/mm (normal range = 0-5 cells/mm ), with a predominance of neutrophils (83%), CSF protein of 435 mg/dL (normal range = 15-60 mg/dL), and glucose < 10 mg/dL (normal range = 50-80 mg/dL).
The infant’s blood and cerebrospinal fluid (CSF) cultures were positive for Gram-negative cocci that were oxidase positive. The neonate was hospitalized, needing mechanical ventilation and vasoactive drugs, and received 21 days treatment with penicillin.
Neisseria meningitidis
Bile salts used on
Gram negative bacilli
Catalase test identifies
Catalase enzyme present? Staphylococcus (+) or Streptococcus (-)
Catalase test method
Drop of H2O2 on sample, O2 release (fizzing and bubbling) is a positive result
Coagulase test identifies
S.aureus (+), coagulase negative staphylococcus
Coagulase test process
The two saline drops are emulsified with the test organism. A drop of plasma is placed on the inoculated saline drop corresponding to test, and mixed well, then the slide is rocked gently for about 10 seconds.
Clumping gives positive result
H2S method identifies
Salmonella (+), Shigella (-)
H2S method
Shows if bacteria can convert sodium thiosulphate to H2S.
Black centres (positive)
Haemolysis test
The types of haemolysis produced by different bacteria are described as alpha, beta or ‘gamma’, if bacteria lyses erythrocytes
If the bacteria cannot grow on bile salts, what type is it?
Haemophilus
Haemolysis test shows
If alpha do optochin test, if beta do Lancefield typing
Optochin test identifies
Used on alpha haemolysing streptococcus
Sensitive - S.pneumoniae
Resistant - viridans, S.mutans
Lancefield typing identifies
A- S.pyrogens
B - S.Agalactiae
D - Enterococcus, S.bovis
Lancefield typing shows
Classifies catalase-negative, beta haemolysing, Gram-positive cocci based on the carbohydrate composition of bacterial antigens found on their cell walls.
Organised into groups
A,B,C,D…
Oxidase test identifies what cocci
Cocci - Neisseria (+), Non pathogenic (-)
Oxidase test identifies what bacilli
Bacilli - (+) Pseudomonas
Oxidase test shows
Determine if a bacterium produces certain cytochrome c oxidases.
The reagent is a dark-blue to maroon color when oxidized, and colorless when reduced.
Oxidase-positive bacteria possess cytochrome oxidase (this oxidases the test strip)
How is lactose fermentation tested?
MacConkey agar
If the bacterium can ferment lactose
(+) red
(-) yellow/clear
Lactose fermentation shows
Tested on gram negative bacilli
If positive do indole test, if negative do oxidase test
Indole test identifies
(+) Escherichia coli
(-)Klebsiella
Indole test
Determine the ability of the organism to convert tryptophan into indole
A positive result is shown by the presence of a red or red-violet color in the surface alcohol layer of the broth. A negative result appears yellow
Urease test identifies
(+) Proteus, negative shigella or salmonella (do H2S)
Urease test
Detects secretion of the urease enzyme, which catalyzes the conversion of urea to ammonia and carbon dioxide.
Yellow (-) to red (+).
Bordetella Pertussis
Gram negative
Whooping cough
Pertussis toxin
B unit binds membrane receptors on upper respiratory tract cells and pancreas, allowing A subunit to catalyse ADP ribosylation of Gi subunit –> Gi subunit remains in GDP bound inactive state –> unable to inhibit adenylate cyclase so increase cAMP - death of upper respiratory tract cells (less ATP available, altered cellular signalling - reduce function of tissue resident macrophages - reduced bacterial clearance)
Clostridium difficile
Gram positive rod
Causes antibiotic associated colitus (diarrhea cause)
Yellow, plaque like lesions in the colon caused by exotoxin
Escherichia coli
Gram negative rod (O-antigen possessing)
Commensal in gut despite containing LPS, (as separated from immune cells by epithelia of gut)
H.pylori gram
Gram negative
H.pylori evasion
LPS O antigen is Lewis antigen, resembling human blood group antigen –> false recognition as self so evade eliciting immune response.
Modifications to Lipid A of LPS cause change in microbe surface charge - prevents binding of cationic antimicrobial peptides and recognition by TLR complexes - lack of macrophage recognition.
Also modification to flagella prevent TLR5 recognition by macrophages - reduce immune cell recruitment.
MRSA
Methilicillin resistant staphylococcus aureus
Gram positive coccus
Some methicillin resistant S aureus strains release Panton valentine leucocidin (production of this toxin makes strains more virulent. i.e caMRSA - community associated)
Pore forming cytotoxin which has 2 subunits (when joined form a pore that leaks cellular contents) –> cell lysis and activates then induces apoptosis of PMNs.
Neisseria Meningitidis
Gram negative diplococcus –> meningitis and septicemia.
Transmission via respiratory droplets, blebbing sheds LPS (bacterial endotoxin) into blood stream
Immunoglobin A protease: Helps bacteria attach to the upper respiratory tract by cleaving IgA
Neisseria meningitidis evasion
Capsule offers resistance to MAC attack by complement (lysis). When capsule KO, serum bacteria falls rapidly.
Factor H binding protein binds human factor H: an inhibitor of the complement factor C3b, thus presence of factor H on the bacteria, reduces opsonizing activity of factor C3b thus reducing the amount of MAC complex that can attack the gram negative bacteria
N.meningitides has a protective bacterial capsule Allows the bacteria to resists phagocytosis by PMNs as the serogroup B capsule mimics the NCAM-1 (neural cell adhesion protein)
Pseudomonas aeruginosa
Gram negative rod, opportunistic, has LasI/LasR and RhII/RhIR (synthase enzymes) which catalyse auto inducer peptide
When threshold (bacterial) concentration reached then LasR binds autoinducer –> go from lag –> log/exponential phase: complex promotes expression of virulence factors to facilitate: cell aggregation, motility and biofilm formation.
Salmonella
Gram negative rod.
T3SS1 –> inject effectors into host cytosol and for entry into small intestine epithelial cells and formation of salmonella containing vacuole.
Acidification of SCV –> T3SS2 expression, required for survival in host cytosol and intracellular replication.
Shigella
Gram negative
Shigella needs TSS3: commensal –> pathogenic –> bacillary dysentery
Salmonella effector proteins
AvrA effector protein –> deubuitinase: ubiquinated I kappa B alpha mixed with AvrA –> caused it to decrease –> Covalent modifications deregulate MAPK and NF kappa B signalling
Shigella effector proteins
Effector proteins
IpaB mediates membrane fusion, IcsA recruits N-WASP for actin polymerisation, Vira cleaves tubulin at head of actin tail
Ospf = T3SS effector, inhibits MAPK and thus proinflammatory pathways
Shigella toxin
Toxins
Translation inhibitors: Shiga toxin in shigella has enterotoxic and cytotoxic properties removes adenine form 28 S rRNA of human ribosomes -enterocyte cell death
Staphylococcus aureus
Gram positive coccus, colonizes skin/mucosa
Superantigen include:
- Enterotoxin –> gastroenteritis (cytokines activates vomit center via ENS)
- Toxic shock syndrome toxin –> toxic shock syndrome (progresses to multiple organ failure and shock)
- Exfoliation toxin –> scalded skin syndrome in children (toxin cleaves desmoglein 1 of desmosomes)
Streptococcus pneumoniae
Gram positive spherical bacterium
Polysaccharide capsule (virulent) inhibits phagocytosis –> allows growth of bacteria until anti-capsular Igs made, and masks cell surface molecules, resists lysis by MAC and mimics host molecules –> virulent
Evidence for gene transfer of virulence factors
Streptococcus pyrogenes
Gram positive coccus
Spe A and SpeC = superantigens, induce inflammation by non-specifically activated T cells and stimulating inflammatory cytokines, responsible for the rash and the toxic shock like syndrome.
Induce human mononuclear cells to synthesize TNFα, IL-1β and IL-6 suggesting –> mediate the fever, shock and organ failure observed in patients with StrepTSS
Spe B = cysteine protease –> degrades Ig, C3b and ECM
M-antigen = contributes to invasiveness through its ability to impede phagocytosis of streptococci by human polymorphonuclear leukocytes (PMNL)
Type specific antibody against the M-protein enhances phagocytosis
Toxic shock like syndrome –> (toxic shock)
UPEC
Gram negative rod
Type 1 fimbriae –> Entails FimH adhesion, tip binds mannosylated uroplakins on luminal bladder surface –> apoptosis of bladder epithelial cells. Lower UTI –> painful urination, pelvic pressure and bladder infection (cystitis).
P fimbrae –> PapG on tip –> binds D galactose D galactose moieties on P blood group antigen of RBCs and urothelial cells after adhered produces a and b haemolysins –> lysis of urinary tract cells –> upper UTI (pyelonephritis)
Vibrio cholerae
Gram negative vibrio
Which bacteria would give acid fast staining?
Mycobacterium tuberculosis
Koch’s four postulates
- The organism must always be present, in every case of the disease.
- The organism must be isolated from a host containing the disease and grown in pure culture.
- Samples of the organism taken from pure culture must cause the same disease when inoculated into a healthy, susceptible animal in the laboratory.
- The organism must be isolated from the inoculated animal and must be identified as the same original organism first isolated from the originally diseased host.
This commensal microorganism has a rigid cell wall, and is capable of causing opportunistic infections of the mouth, gut and vagina. Infections are more common in people with HIV infection.
Candidas albicans
This filamentous fungus produces spores that can trigger a type III immune pathology in the lung of sensitized individuals due to the build up of antigen/antibody complexes that trigger complement activation.
Aspergillus clavatus
This single celled pathogen causes a form of pneumonia in immune compromised individuals that is refractory to treatment with anti-bacterial antibiotics and used to be the leading cause of death in HIV/AIDS patients.
Pneumocystitis
In the small intestine, cholera toxin acts by
ADP-ribosylation of Gs which makes it constitutively active which results in phosphorylation and activation of Cl- channels
Which type of bacterial toxin is being described in the following sentence? “These toxins indiscriminately activate T cells of the immune system causing system-wide inflammation and other serious, potentially fatal, symptoms.”
Superantigen / Endotoxin
A lipid found in high concentration in the cell wall of acid-fast bacteria.
Mycolic acid (peptidoglycan-arabinogalactan-mycolic acid wall structure)
A protein filament that facilitates the attachment between bacterial cells during conjugation.
Sex Pilus
