Infectious diseases Flashcards
Where does the majority of the bodys normal flora exist
Large intestine >10 to the power of 14
Where are the other sites of the bodys flora
Skin
Oral cavity
What type of bugs occupy the skins normal flora
Majority are obligate anaerobes
What are the 4 types of common flora
- bacteria (i.e. prokaryotic cells)
- fungi (e.g. yeast Candida albicans and malassezia furfur)
- viruses (e.g. herpes group)
- possibly parasites (e.g. intestinal amoebae)
Why is the skin not an ideal environment for many microbes
- resident microbial flora
- dryness
- acid pH (~5.5)
What type of environemnt in the body fo flora microbes predominate?
High humidity/ nutrient rich areas
What microbiral flora is found in the upper intestinal tract
Enterococcus faecelias
What anaerobes are predominantly found in the mouth
Porphyromonas, Prevotella and Peptostreptococcus genera
What antibiotics are anaerobes in the mouth susceptible to
are susceptible to amoxycillin/clavulanic acid (coamoxyclav) combinations.
What abx are good for bite wounds and why
Augmentin (anaerobes in mouth are sensitive to this)
Is candida albicans a common skin commensal?
What circumstances allow it to colonise?
No
Excess hydration (i.e nappy rash) can allow to it transiently colonise
What is an important difference between gram positive and gram negative cell walls.
Gram positive has a thick cell wall with NO outer membrane.
Gram negative has a thin cell wall with an outer membrane
What are the 3 important features of the outer membrane of the gram negative cell wall
LPS (lipopholysaccaride which is made up of the following)
- lipid A classical endotoxin responsible for toxicity
- polysaccharide core
- terminal repeating units of ‘O’ somatic antigenic side chains, both important taxonomically
What is the function of the gram negative outer membrane
- Prevents leakage of periplasmic proteins
- Protects the cell (in enteric bacteria) from bile salts and hydrolytic enzymes of host environment
- Proteinaceous pores (porins) -> allow passage of LMW solutes (large antibiotic molecules penetrate it slowly)
- may act as a barrier to the entry of antimicrobials, as most antibiotics are not lipid soluble, so can’t get through OM
What does the cell wall of gram positive bacteria consist of
- Peptidoglycan containing strands of tecihoic acid, proteins and CHO (depending on the species)
- Not protected by an outer membrane (therefore susceptible to degradation by lysozyme)
How does the cell wall of mycobacteria differ from both gram positive and gram negative bacteria
contains large amounts of lipid , responsible for intracellular survival, adjuvant effect and survival in presence of acid
What happens when Gram negative LPS is injected intravenously
- produce arteriolar dilatation
- activate complement through the alternate pathway
- injure endothelial cell membrane
Under optimal conditions how fast can bacteria divide
10 to the power of 8 progeny in less than 24 hours
How do bacteria divide and how often
Binary fission every 10-20 minutes in vitro
What conditions do aerobic bacteria divide in
need air (i.e. oxygen as the hydrogen acceptor) to grow and divide
What conditions do anaerobic bacteria divide in
Only grow in the abscence of O2
What happens to obligate anaerobes exposed to oxygen
sensitive to oxygen, gets killed after brief exposure
What conditions do facultative anaerobes grow in
Anaerobe or aerobically
What conditions do microaerophilic microbes grow in
grow best under slightly reduced oxygen levels
What 3 ways are genes transferred between cells
Transformation
Transduction
Conjugation
Explain how transformation works and which bacteria is is common in
- recipient cell takes up DNA released from donor cell
- common in pneumococci, meningococci
Explain how transduction works and which bacteria it is common in
DNA transferred from donor cell to recipient via a virus (bacteriophage) vector
- common in staph
Explain how conjugation works and which bacteria it is common in
transfer always from donor to recipient via the fertility (F) factor, typically a plasmid or transposons (movable
segments of DNA)
What is a plasmid
Extra-chromosomal fragment of DNA
usually circular
5 reasons why plasmids are important
- may be a/w virulence
- are important in the transfer of drug resistance
- readily transmissible between related and unrelated bacterial strains
- often carry genes responsible for virulence and antibiotic resistance (e.g. resistance or R plasmids)
- whole or parts of plasmids may incorporate into chromosomal DNA
4 facts about genes for antibiotic resistance
- are often found in plasmids
- may be incorporated into cosmids
- often occur in circular DNA molecules
- are useful markers in recombinant DNA technology
What are the 2 types of toxins in bacteria
Endotoxin
Exotoxin
Which are more potent, endotoxins or exotoxins and why
exotoxins, they have a more specific site of action
Where do exotoxins produce their effect
Locally (may be severe eg gas gangrene)
OR distant from site of entry e.g Tetatnus
Which bacteria secrete exotoxins (be specific)
- Corynebacterium diphtheria
- clostridium difficile
- staph. aureus )staph enterotoxins)
- pseudomonas aeruginosa (one of the few gram negatives to excrete a significant exotoxin)
- tetanus toxins
Are exotoxins proteins or lipids
Proteins
Are endotoxins proteins or lipids
Lipids
Lipid A portion of LPS!
When are exotoxins secreted
During growth of may gram positive bacteria