Concepts in microbiology - bacteria Flashcards
Which pathogen causes angular cheilitis?
Staphylococcus aureus
2 possible sources of Staphylococcus aureus
Endogenous (originates internally, within the body) or exogenous (originates externally and is passed on by cross infection)
How is Staphylococcus aureus passed on?
Contaminated hands and fomites
Oral diseases caused by Staphylococcus aureus
Angular cheilitis, mucositis, bone infection
Symptoms of angular cheilitis
red, inflamed, crusty angles of mouth
What does a triangle symbolise?
diagnosis
How would you sample an oral infection (e.g. angular cheilitis) to send to the microbiology lab?
Using a sterile swab that is moistened with sterile water
What is the diagnostic process used in the microbiology lab to identify the bacteria?
- grow on selective and non-selective agar
- observe colony morphology
- Gram stain
- clumping factor detection
What is the name of the non-selective agar?
Blood agar
Why is bacteria grown on non-selective / blood agar?
To observe the colony morphology (features used to identify). All bacteria grow giving an overall impression of bacteria flora present.
What is the colony morphology of Staphylococcus aureus on non-selective / blood agar?
Round, convex, smelly
Appearance of Staphylococcus aureus on a gram stain
Dark clusters resembling grape bunches
Why is Staphylococcus aureus the only species able to grow on selective agar?
The specific salt concentration inhibits growth of normal flora but allows Staphylococcus aureus to grow.
How does selective agar change colour in the presence of Staphylococcus aureus?
Selective agar contains the sugar mannitol which is fermented by S. aureus. Acid released which lowers the pH causing the pH indicator in selective agar to turn from pink to yellow.
Example of a selective agar
Mannitol salt agar
Why does Staphylococcus aureus cause coagulation of the serum in the Coagulase test?
S aureus has the coagulase positive virulence factor
Instead of adding inoculum of bacteria to a test tube of serum, which method is now used for the coagulase test?
Slide agglutination test
Why does Staphylococcus aureus cause pus collection?
it has a coagulase positive virulence factor
How does the Slide agglutination test work? (replacement of coagulase test)
The clumping factor and protein A is found on the S aureus cell wall. Fibrinogen and IgG (antibody) is on the latex bead surface. S aureus clumps in the latex bead solution and is positively identified.
Example of coagulase positive staphylococci
Staphylococcus aureus
Example of coagulase negative staphylococci
Staphylococcus epidermis (commensal bacteria found on skin)
Meaning of ‘staphylos’ ‘coccus’ ‘aureus’
Staphylos - grapes in Greek
Coccus - round shape
Aureus - golden colour in blood agar
Why is antimicrobial susceptibility testing conducted?
Inform which antibiotic should be used and for surveillance of antimicrobial resistance
How is susceptibility / resistance to antibiotics for bacteria deduced?
Bacteria incubated on agar plate with antibiotic disks. Measure zones of inhibitions. This process is automated by Vitek machine
What is the minimum inhibitory concentration susceptibility test?
A quantitative measure of the concentration of antibiotic required to inhibit the bacteria. An E test strip is placed on an agar plate coated in bacteria and increasing concentrations of the antibiotic.
Why is it important to take specimens for identification and susceptibility testing?
Staphylococcus aureus can cause severe head and neck infections. Particular strain of S aureus produces an exotoxin which is associated with community acquired MRSA (methicillin resistant Staphylococcus aureus)
Size of bacteria
0.5 - 5 um (1um = 1000 nm)
Features common to prokaryotes and eukaryotes
cell membrane and ribosomes (70s vs 80s)
Difference between prokaryotes and eukaryotes
prokaryotes have circular DNA in nucleoid (space where DNA is found) while eukaryotes have linear DNA in the nucleus. Prokaryotes do not have membrane-bound organelles. Ribosomes in prokaryotes are smaller (70s) than eukaryotes (80s)
Why is it important to understand differences between host and bacterial cells?
Insight into disease processes (e.g. ecological plaque hypothesis), disease diagnosis (e.g. pulper exposure due to caries caused infection) and disease treatment (e.g. mouthwash, toothpaste)
Features of a bacterial cell
All: cell envelope, cytoplasmic membrane, 70s ribosomes, circular DNA
Some: membranous invaginations, inclusion granules, flagellum, capsule
Why is attachment of bacteria to host important?
If the bacteria doesn’t attach, it cannot survive long
How do bacteria attach to host cells?
using pili (fimbriae)
Which bacteria have pili?
Gram negative
How are pili specific to the host and the tissue?
Pili bind to specific host glycolipids / glycoproteins
Virulence definition
Ability of microorganism to cause damage to host
Virulence factor meaning
factors that contribute to virulence i.e. factors that determine extent of disease caused by pathogen
Examples of virulence factors
Pili / fimbriae and flagella
How do fimbriae / pili contribute to virulence?
Allow adhesion and are antiphagocytic
How do flagella contribute to virulence?
Allow for chemotaxis (movement of organism in response to chemical stimuli) and penetration of mucus (not trapped)
Aerobic (bacterial aerobes) meaning
need oxygen
capnophilic meaning
CO2 required for growth
Facultative definition
Can survive with or without oxygen (e.g. Staphylococcus aureus) by switching on/off different metabolic pathways
Anaerobic definition
Survive without oxygen (toxic to them). Metronidazole targets bacterial anaerobes
Microaerophilic definition
require small amounts of oxygen
How do bacterial colonies grow?
Divide by binary fission - exponential growth
Stages in a bacterial growth curve
Lag, log, stationary, death phase. knowledge of death stage especially important.
Why is it important to understand bacterial growth kinetics?
insight into disease treatment and for pharmacodynamics of antibiotics
Series used to sterilise dental instruments
Pretreatment - air removed by fractioned prevacuum
Sterilisation - holding time at 134 degrees C
Posttreatment - drying by postvacuum.
Demonstrated on a pressure-time graph
How is equipment kept sterile in the dental practice?
Sterile packs and wraps for handpieces. Disinfectants used e.g. perform for impressions to prevent bacterial cross infection.
How do you visualise bacteria?
Not visible on a glass slide under microscope.
Use gram stain
Stages of gram stain reaction
- make a film
- flood with crystal violet
- all cells take up purple dye
- fix crystal violet to cell wall by flooding with Lugol’s iodine (fixative)
- all cells appear blue-black
- decolourise with acetone/alcohol
- gram positive retain dye (purple), gram negative decolourise
- Counterstain with red dye e.g. safrinin
Gram +ve = blue-black
Gram -ve = red
What colour are gram positive bacteria in a gram stain?
Blue-black e.g. S aureus
What colour are gram negative bacteria in a gram stain?
Red
Why does crystal violet not stick to gram negative bacteria?
Gram negative bacteria have a thinner peptidoglycan layer - their cell wall is made of lipopolysaccharide mainly
Why does crystal violet stick to gram positive bacteria?
Gram positive bacteria have a thicker peptidoglycan layer in their cell wall
How can gram staining indicate which antibiotic to use?
Penicillin can target gram positive bacteria
What feature of gram positive bacteria is targeted by penicillin?
Penicillin Binding Proteins which help cross link peptidoglycan side chains are targeted. Prevents further crosslinking of peptidoglycan side chains so bacteria are less resistant to osmotic pressures therefore cell walls more likely to lyse.
What is an exotoxin?
Toxin produced by both gram positive and gram negative bacteria
Exotoxin example
Leukocidin is an exotoxin released by PVL positive Staphylococcus aureus
Mode of action of Leukocidin
S aureus releases 2 components of Panton-Valentine Leukocidin (PVL). PVL assemble into a pore-forming heptamer on Polymorphonuclear white cell (PMN) membranes. High PVL concentration causes PMN lysis. Creates an inflammatory response and eventually leads to tissue necrosis.
A more common term for polymorphonuclear white cells (PMN)
pus cells
What are endotoxins?
Lipopolysaccharides that are part of the outer membrane of gram negative bacteria
Components of lipopolysaccharide
O antigen, core, lipid A (drops into cell membrane)
Example of an endotoxin/lipopolysaccharide
Prevotella intermedia
What is the localised oral disease caused by Prevotella intermedia (endotoxin)?
Chronic periodontitis
Potential trigger of sepsis
Endotoxin
Different bacterial shapes
cocci (round), bacilli (rod), spiral shaped
Example of a gram positive cocci bacteria
Streptococcus mutans (caries, grows in chains), Staphylococcus aureus
Example of a gram positive bacilli bacteria
Clostidium tetani (tetanus)
Effect of Clostidium tetani exotoxin
causes tetanus and spinal contractions
Example of gram negative cocci bacteria
Neisseria meningitidis (prevention: meningococcal ACWY vaccine)
What is the differential diagnosis of meningococcal septicaemia?
Glass tumble test
Example of gram negative bacilli bacteria
Prevotella intermedia (periodontal disease)
What oral disease is caused by spiral shaped bacteria?
acute (necrotizing) ulcerative gingivitis
Which bacteria is an exception to the gram stain?
mycobacteria (gram stain cannot penetrate thick waxy cell wall)
Which bacteria is a rapidly growing, nontuberculous mycobacterium (NTM), found ubiquitously in the environment in water, soil and dust?
Mycobacteria abscessus - caused infection after pulpotomy by contaminated water.
What alternative staining is used for mycobacteria?
Fluorescent stain, auromine staining
