introduction to infectious agents

Question 1

what is the resolving power of the eye and a light microscope

Answer 1

resolving power of the naked eye - 100um

resolving power of a microscope - 1.2um

Question 2

what are the basic characteristics of bacteria

Answer 2

Genus/species – Bacteroides fragilis
Light microscope -1um diam./ 3-10um length
Procaryotic – divides by binary fission
Doubling time – 20 min for E.coli
Cell wall differentiates – Blue/purple (positive), red (negative)

Question 3

what is a pathogen

Answer 3

organisms that causes or is capable of causing a disease

Question 4

what is a commensal

Answer 4

organism which colonises the host but causes no disease in normal circumstances

Question 5

what is a opportunist pathogen

Answer 5

microbe that only causes disease if host defences are comprimised

Question 6

define virulence / pathogenicity

Answer 6

the degree too which a given orgnanism is pathogenic

Question 7

what is asymptomatic carriage

Answer 7

when a pathogen is carried harmlessly at a tissue site where it causes no disease

Question 8

what are areas open to bacterial colonisation called

Answer 8

mucosal surfaces

Question 9

how does bacteria form

Answer 9

shape size and aggregation , cocci , rods clumps , chains and pears Aggregation in relation to bacteria refers to the process of bacterial cells coming together to form aggregates or clusters

cell wall structure - gram positive and gram negative

other structures, capsule , spore , flagella pilli

Question 10

what does gram positive and gram negative refer to

Answer 10

Gram-positive and gram-negative are two categories of bacteria that are distinguished based on their response to a laboratory technique called the Gram stain. The Gram stain is a differential staining technique that can be used to visualize the cell walls of bacteria.

Gram-positive bacteria have a thick peptidoglycan layer in their cell walls that stains purple when exposed to the Gram stain. This thick peptidoglycan layer also helps to retain the crystal violet-iodine complex used in the staining process, giving the bacteria a purple color under a microscope.

Gram-negative bacteria have a thinner peptidoglycan layer in their cell walls and an outer membrane that contains lipopolysaccharides. During the Gram staining process, the outer membrane is dissolved by the decolorizing agent, causing the crystal violet-iodine complex to be washed out of the cell. The bacteria are then counterstained with a red dye, which stains the remaining peptidoglycan layer and gives them a pink color under a microscope.

In summary, gram-positive bacteria have a thick peptidoglycan layer in their cell walls that stains purple during the Gram stain, while gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that stains pink during the Gram stain. The Gram stain is a useful diagnostic tool in identifying and classifying different types of bacteria.

Question 11

how does bacteria grow

Answer 11

agar medium media ,(broth)

aerobic (grow in oxygen) anaerobic grow without oxygen, facultative (either)

shape of colony , diameter , edge , domes or flat

effect on medium- heamolylis (streptoccus), pigment production , pseudomonas , acid from sugar , egg, lactose (esteriochia cali)

Question 12

how are antibiotics separated into groups

Answer 12

Many groups, split on chemical structure and mode of action such as penicillins and tetracyclines
Can be either bactericidal or bacteristatic
Activity determined by Minimum Inhibitory Concentration (MIC) – the lower the MIC the more active the drug
In clinical use since the 1940s

Question 13

what is antibiotic resistance

Answer 13

Developed soon after antibiotics used in treatment
Most common mechanism is via enzyme – beta-lactamase (ESBL)
Widespread and often indiscriminate use has led to multiresistant organisms (selection)
Several bacteria now very resistant (MRSA) and difficult to treat
Greater reliance now on infection control and/or vaccines

Question 14

What is the size of bacterial chromosome

Answer 14

Bacterial chromosome (ds DNA) typically 2-4 x 103 kb

Question 15

how is genetic variation created In bacteria

Answer 15

Mutation
Base substitution
Deletion
Insertion
Gene transfer
Transformation eg via plasmid
Transduction eg via phage
Conjugation eg via sex pilus

Question 16

what is the prevlanace of healthcare associated infections

Answer 16

300,000 HAIs per year in UK
Costs NHS £1 billion
Major impact on mortality and morbidity increasing length of stay and cost
Increasing problem in community settings such as nursing and residential homes

Question 17

what hygiene measures are put in place to reduce the spread of mrsa

Answer 17

Clean hands before and after touching patients
Hands cleaned with soap and water, or alcohol gel or hand rub
Wear gloves and aprons when caring for a patient with MRSA
A patient with MRSA may be moved to a room on their own or into a separate area for people who have MRSA or other infections

Question 18

how to avoid getting c . difficile

Answer 18

Wash hands thoroughly with soap and water before preparing/eating food, after handling raw food, after going to the toilet, after visiting hospitals and care homes
Take antibiotics only when necessary
Wash all dirty clothes, bedding, towels of infected patients in washing machine on hottest cycle. Clean toilet seats, flush handles, taps, after use with detergent and hot water

Question 19

what is chalamydia trachomatis

Answer 19

Related to Chlamydophila pneumoniae and C.psittaci
Obligate intracellular bacterium
Produces Elementary and Reticulate bodies (EBs/RBs) can be seen by light microscope
Unique developmental cycle

Question 20

where is chlamydia trachomatis detected

Answer 20

Grown in tissue culture
Detected by molecular methods
Divided into 3 groups:
Serovars A-C = Trachoma
Serovars D-K = NGU/ Conjunctivitis
Serovars L1-L3 = Lymphogranuloma venereum
Treated with tetracycline, azithromycin

Question 21

what Are the basic characteristics of viruses

Answer 21

DNA (Herpes) or RNA (HIV)
Require electron microscope to be seen (25 – 150nm)
Several morphological forms
No metabolic systems – requires tissue culture

Question 22

how do viruses grow

Answer 22

Many different types of cells used for culture – growth determined by cytopathic effect (CPE)
Growth recognised by unique appearance e.g. adenonvirus –grape-like clusters
Also by antibody neutralisation and erythrocyte agglutination

Question 23

describe the structure of viruses

Answer 23

Virion: the intact virus particle
Capsid: the protein coat
Capsomeres: the protein structural units which make up the capsid
Nucleic acid genome: either DNA or RNA

Question 24

how is bacteria diagnosed in labs

Answer 24

CPE
Electron microscopy - morphology
Serology – antibody tests e.g. ELISA looking for >4 fold rise in titre
Molecular tests such as PCR and commercial DNA/RNA detection systems

Question 25

what is protozoa

Answer 25

Eukaryote (3-2000um)
Contains nucleus, no cell wall
Unicellular, often in soil and water
Life cycle, trophozoite and cyst stages
Examples, Acanthamoeba, Toxoplasma gondii

Question 26

what are bacteria and viruses detected by

Answer 26

Although both detected by NAATs, otherwise different diagnostic approaches for bacteria and viruses

Question 27

what is the resolving power of the eye

Answer 27

0.1 of a mm is the resolving power of the naked eye

the resolving power of a light microscope is 0.2 microns

microbes are smaller than what we can see with the naked eye

Question 28

what are the different types of microbes

Answer 28

eukaryotic or protozoa types of microbes

protozoa microbes are the large ones and the bacteria are in the middle in terms of size and the bacteria are the small ones

Question 29

describe bacteria morphology

Answer 29

bacteria are single cells

some of the are rod shaped which are called bacilli

and some are round shaped which is called coci

they have naked dna in the cytoplasm - i.e. so they don’t have a nucleus but they have chromosomes

depending on what types of bacteria they are they can either have one or two membranes

they can have organelles such as flagella for motility

and pilli which help them stick to surfaces

like eukaryotic cells they have ribosomes- dna is transcribed into rna and then ribosomes translate the rna into proteins

Question 30

how is dna replicated in bacteria

Answer 30

you have naked dna where the genes are found

the chromosomes are transcribed into rna

and then the ribosomes translate the rna to into proteins

Question 31

how does bacteria have different morphology

Answer 31

bacillus

comma shaped ones called vibrios

spiral ones e.g. campylobacter

trepaneama

slightly spiral ones such as campylobacter

round ones Called coci which form in clusters/ chains

or ones in pairs that are called diplococci

Question 32

what are some basic characteristics of bacteria

Answer 32

genus - and species name e.g. bavcteroides fragility in coli

genus = bacterioides

genus =fragillis

seen with light microscopes - 1 diameter

and 3-10 diameters in length

and they divide by binary fission - the length of time for this dividing can vary greatly

for ecoli its doubling time is every 20 minutes

Question 33

what does gram positive and gram negative mean

Answer 33

if you have 1 cell membrane you are gram positive

and if you have 2 cell membranes you are gram negative

the gram stain helps to differentiate the different types of bacteria

Question 34

if you have a gram stain of blue what does it mean

Answer 34

blue/ purple = gram positive meaning you have 1 cell membrane

red (negative) = 2 cell membranes and it means that you are gram negative

Question 35

what areas are open to bacterial colinisation

Answer 35

muscosal surfaces

the bladder and the kidney are kept sterile

the gall bladder

the lungs e.g. infection of lungs pneumonia

if you get bacteria in these areas the body sets up a large inflammatory response

Question 36

why are capsules important

Answer 36

capsules are important for bacteria that get into the blood system and into the lungs because these help to protect them from the inflammatory response and being phagocytoses by white blood cells - made from polysaccharides

one of the organisms that cause sepitsemia and meningitis is neisseirhia meningitis - can cause inflammation of meningitis - lining of the brain

Question 37

describe a gram positive bacteria

Answer 37

stains purple - has a single cell membrane

connected to the single cell membrane = peptidoglycan - bacterial skeleton on the outsdie of it - this forms a rigid structure on the outside of the bacteria - on the outside of this you can have a capsule - peptidoglycan is connected to the cytoplasmic membrane through leptitoechic acids

Question 38

describe gram negative bacteria

Answer 38

they have 2 membranes - inner and outer between the two space called the periplasmic space and in this area = thin layer of peptipdyclan

has a smaller skeleton between these two membranes

connected to the outer leaflet of the gram - outer membrane = lipopolysaccharide - connected to outer leaflet and extends out

o antigens and lipid a present

lipopolysaccharides is also called endotoxin

capsules also present to protect it from it from the immune system

Question 39

if there was a picture of round purple bacteria what would it indicate

Answer 39

gram positive cocci

in clusters

Question 40

if there was a picture of purple rods what would it Indicate

Answer 40

• basilar

gram positive rods

Question 41

if you had a picture of pink rods what would it inidicate

Answer 41

pink shaped rods would indicate gram negative rods

in some cases you can have gram positive rods which have terminal rods on them - these are spores - spores tend to be produced by gram positive organisms to help them survive harsh enviro conditions

Question 42

how is bacteria grown

Answer 42

agar medium utlisiing the nutrients within the media

or broth

aerobic conditions or aerobic conditions - some bacteria are poisoned by presence of oxygen

facultative either e.g. coli ( can grow in aerobic envriometns or aerobic)

when bacteria grow on agar plates they form colonies

some bacteria is grown on agar plates that have blood on them and are able to produce toxins which lyse open blood cells and cause heamolysis

some bacteria grown on agar plates causes pigment production e..g pseudomonas

some grow on different sugars and they ferment the sugars and produce and acid e..g escherirchia coli

Question 43

how are different bacterias diagnosed

Answer 43

using lots of different agar plates with different infdciatirs and substrates in them

selective agar plates

you can grow different colonies and put them on a api test strip

each strip tests for a change of colour - can differentiate bacteria through there different bicochemical reactions - particular organisms have a particular biochemical fingerprint

Question 44

what are different biochemical tetsts

Answer 44

catalase - mix hydrogen peroxide with bacteria on a loop - produces oxygen

catalase positive - oxygen and bubbling

oxidase agent- oxidase positive - purple

one of the tests for staphloccocus is mixing staph with serum to do a coagulase test - staphloccosus is coagulase positive - causes the serum to form into a help - it coagulates the jelly

mix serum and bacteria if it forms emulsion = coagulase -

if it begins to clump= coagulase +

Question 45

what are both staphylococci and sterptococci

Answer 45

gram positive cocci