Introduction - The Nature of Infection Flashcards
List different types of micro-organisms/infectious agents in order of decreasing size
Parasites Fungi Bacteria Viruses Prion Proteins
Describe the naming of organisms
Organisms have 2 names (in italics):
GENUS name - 1st letter is UPPER CASE
SPECIES name - 1st letter is LOWER case
What are prion proteins? Describe what they do and how they can be removed
Rare infectious agents that have no DNA/RNA; cause an incurable brain infection (brain matter lost, appears spongy)
Cannot be reliably removed by sterilisation/disinfection so surgical instruments are destroyed after use on such patients
Responsible prion disease like “mad cow disease” (AKA CJD - Creutzfeldt-Jakob Disease)
What are viruses? Describe their structure
Very small, obligate, intracellular parasites that contain either RNA or DNA
Can only be seen using an electron microscope
They have a protein coat (capsomere) and/or a lipid envelope
What are the methods used to diagnose a virus?
Cell cultures - no common now
Serology - antigen or antibody detection in blood
PCR - to detect the DNA or RNA. Important in giving real-time results as faster than cell culturing
Describe bacterial structure and contents
Can be seen with a light microscope
Contain both DNA and RNA and have a single double-stranded chromosome
They can have plasmids (extra pieces of circular DNA that often code for antibiotic resistance)
Have a cell membrane and cell wall and/or a capsule
The flagellae allow movement of the cell
The pili (fimbriae) allow adhesion of cell to surfaces
What are the different bacterial shapes?
Coccus (round)
Bacillus (rod-shaped)
Spirochaetes (spirals)
What is Gram Stain?
Stains the bacterial cell wall
Gram positive - purple (bacteria have a thicker peptidoglycan wall)
Gram negative - pink (thin bacterial cell wall so do not hold onto stain well)
Both can result from coccus or bacillus bacterial cells
Importance of gram stain
Allows guessing of the most likely organism
Also, many antibiotics act on cell wall so this is important in determining what antibiotic to use
Gram stain limitations
Not all organism stain well: Mycobacterium tuberculosis has a waxy coat that does not take up stain Treponema pallidum (spirochaete organism; causes syphilis (STD)
Other staining method must therefore be used for these infections
What are commensal bacteria and give examples of regions where they are found
Normal bacterial flora that usually not harmful, if in the correct region
Upper respiratory tract
Lower small intestine and large bowel
Vagina
Skin - resident and transient flora (hand-washing effectively rids the transient flora)
Damp areas (armpits, groin) have rich flora
What is a pathogen? What is an opportunistic pathogen
A harmful organism
An organism that will probably only cause infection in an immunocompromised individual (e.g: due to immunosuppressants)
What is meant by pathogenicity?
Ability of a microorganism to cause disease
What is meant by virulence of a pathogen?
The degree of pathogenicity of an organism (how easily it causes disease)
How do bacteria replicate and what do they require?
Via binary fission. Require:
Food (any organic material)
Correct temp (37 C for most human pathogens)
Correct pH (7.4 for most human pathogens)
How does binary fission occur?
Bacteria undergo nucleic acid replication
2 sets of chromosomes go to opposite sides of cell
Cell wall forms, dividing cell into 2
Bacteria double
What are the phases of bacterial growth?
Lag phase (1st part of curve) Log phase (exponential growth - nutrients available) Stationary phase (rate of death = rate of regeneration; nutrients not abundant) Death phase
What kinds of atmosphere do bacteria grow in?
Aerobic
Microaerophilic
Anaerobic
What are aerobic bacteria?
Grow in the presence of oxygen
Many will also grow anaerobically but not as well
What are microaerophilic bacteria?
Grown in an atmosphere with reduced oxygen concentration with enriched with CO2
What are anaerobic bacteria?
Only “strict” anaerobes
Grow in an atmosphere of no oxygen
What toxins do bacteria produce?
These are enzymes
Exotoxin (mainly gram positive bacteria) - produced inside cell and exported from it
Endotoxin (mainly gram negative bacteria) - part of the outer membrane of the cell wall and usually only released for membrane when cell dies
What are some effects of bacterial toxins?
Interact with immune cells causing release of cytokines
Damage red and white blood cells, leading to “leaky” small blood vessels; causes decreased BP and affects blood clotting.
Leads to sepsis and septic shock
What are spores?
Inactive, hardy forms of bacteria that cannot replicate but can survive adverse conditions for many years (they are thick coats containing DNA)
Produced by some gram positive bacilli
Steps of spore formation
Chromosome condensation and septum formation
Calcium dipicolinate accumulates in the core of the developing spore
How are bacteria classified?
Gram positive/negative
Atmosphere in which they grow
Shape (bacilli/cocci)
Other: biochemical tests, coagulase, types of haemolysis on blood
Bacteria diagnostic methods
Microscopy Culture Detection of antigen Detection of antibodies in blood PCR NAATS - (Nucleic Acid Amplification Tests) detection of organism's DNA/RNA
Describe steps in bacterial cell culture
Culture samples on agar plates to allow bacterial growth
Use specialised tests to help identify organism, like biochemical reaction
Provide info on which antibiotics will be most effective:
Sensitive (S) - can be used to treat infection
Resistant (R) - cannot be used to treat infection
Gradually replaced by PCR
Length of time for bacteriology tests
Microscopy - same day
PCR - same day (sometimes batched)
Agar culturing and antibiotic sensitivity tests - 48 hrs (24 to grow organism and 24 for sensitivity)
Serology - same day (often batched)
TB culture - 4-12 weeks (can be detected using PCR faster)
Give examples of fungi and what they do
Moulds - complex structures that produce spores (spread in air currents) and hyphae (filaments that invade organic tissue). Do not stain with gram stain. Most common cause of fungal infection is Aspergillus spp
Yeast - single cell reproduce by BUDDING - not binary fission (mother cell gives rise to several budding daughter cells). Most common cause of fungal infection is Candida spp. Stain as large gram +ve oval structures
Why are antibiotics not used to kill fungi?
Cell wall different from bacteria.
Special anti-fungal drugs used
Describe parasites and their structure
Complex interaction with the host - “life cycle”
Structures are varied, from single-celled organisms (protozoa) to more complex organisms
Give an example of a parasite with a simple life cycle and a complex life cycle
Giardidis - human host only
Shistosoma - worm whose life cycle involves humans and snails
What is sterilisation?
Destruction and removal of 99.9% of microorganisms and spores (may not inactivate prion proteins)
What are the methods of sterilisation?
Steam and pressure (autoclaves)
Dry heat in oven at 160C for over an hour
Exposure to ethylene oxide gas - explosive so must be done carefully
Gamma irradiation
What is disinfection? What are powerful and mild disinfectants used for?
Removal/destruction of PATHOGENIC microorganisms to make item safe
Powerful - medical equipment that does not need to be sterile
Milder - hands, patient’s skin (antispectics)
Methods of disinfection?
Hot Water
Chemical: bleach and handwash
