Topic 6- Infection, Immunity, and Forensics

inhalation

breathing in lipid droplets containing the pathogen previously expelled from the respiratory system of an infected individual

direct contact

skin to skin contact and maternal transmission from mother to unborn child or in breast milk

inoculation

pathogen entering directly into the body through break in the skin

vectors

living organism that transmits an infection from one host to another

ingestion

contaminated food or drink leads to vomiting and diarrhoea- greatest risk from raw food or recontamination of cooked food

fomites

inanimate objects that carry pathogens from one host to another

skin as a barrier

contains tough protein keratin but can be breached by wounds which can be helped by blood clotting

skin flora outcompete and prevent colonisation by other bacteria

mucosal membranes

line the airways the gut- mucus traps microbes and other particles

beating cilia carry the mucus up to the throat to be swallowed

secretions from eyes and nose contain the enzyme lysozyme that breaks down bacterial cell walls causing lysis

stomach acid

HCl pH less than 2 that kills most bacteria that enter with food or drink

pH also optimum for the digestive enzyme pepsin

gut flora

bacteria found in intestines that have a mutualistic relationship with animals as they aid digestion and outcompete pathogenic bacteria for food and space

gut flora also secrete lactic acid to aid defence against pathogens

fate of bacteria if non-specific immune response doesn’t work

get carried away from site in blood or lymph and travel to lymph nodes where they undergo phagocytosis by macrophages

if this fails there is widespread infection leading to septic shock

inflammatory response

damaged basophils and mast cells release histamine causing vasodilation of arterioles to increase blood flow to site- becomes red and hot

histamine also makes capillaries more leaky so plasma phagoctes and antibodies can enter the site, causing oedema

lysozyme

found in tears nasal secretions and saliva- break down bacterial peptidoglycan cell wall allowing water to enter the cell and causing it to burst and be destroyed

interferon

provides defence against viruses but also some bacteria

released from infected cells and diffuses into surrounding cells where it prevents viral protein synthesis and therefore replication

active natural immunity

an individual mounts a specific primary immune response after naturally contracting a pathogen and creates antibodies and memory cells for long term protection

passive natural immunity

antibodies from one individual are passed to a second individual in a natural process for short term protection eg from mother to baby via the placenta

passive artificial

antibodies from one individual are passed to another individual via an injection to provide immediate protection for short term protection

active artificial immunity

a form of pathogen is injected into the individual who then mounts a specific primary response to create antibodies and memory cells for long term protections

vaccination

must contain one or more antigens that are also found on the pathogen or the toxin

attenuated virus vaccination

weakened viruses that are harmless and non-pathogenic and less virulent. they will reproduce very slowly but not result in symptoms in the host

killed bacteria vaccination

the bacteria have been killed by chemicals, heat, or radiation

harmless toxin vaccination

inactivated toxic compound that causes an illness rather than the microorganism that produces it

antigen bearing fragmemt vaccination

newer form- can consist of the surface protein of a microorganism

herd immunity

when enough people in a population are successfully vaccinated and immunised, the pathogen is less likely to be transferred from one person to another which reduces the incidence of the disease in the community and protects those not able to be vaccinated

evolutionary race

as soon as the host evolves mechanisms to combat the pathogen, the pathogen evolves new methods of overcoming the immune system of the host. the host antibodies and memory cells are now useless

why do bacteria evolve so quickly

• reproduce very quickly

• large populations so rate of mutations is vast, and there is a very large gene pool

• some random mutations will be advantageous

the host immune system is the selections pressure

bacteriacidal antibiotics

antibiotic that destroys/kills bacteria

bacteriostatic antibiotic

antibiotic that inhibits the replication of bacteria which alllows the host immune system to destroy the bacterial population

antibiotics mechanisms that can disrupt bacterial cell division

• inhibition of cell wall synthesis

• disruption of the cell membrane

• inhibition of nucleic acid synthesis, replication, or transcription

• inhibition of protein synthesis

• inhibition of specific bacterial enzymes

process of bacterial resistance to antibiotics

• presence of antibiotic acts as a selecion pressure

• there is some naturally occuring genetic variation in the bacterial population where a random mutation creates an allele for resistance

• the bacteria that don’t posess the allele are killed but those that do are more likely to survive and reproduce

• the frequency of the advantageous allele will increase and can be passed via vertical evolution to offspring or horizontal evolution to other bacteria via conjugation

codes of practice

• handwashing

• doctors and nurses told to not to wear watches, ties, and long sleeves

• regular deep cleaning and changing of patient gowns and bedding

• screening and isolation of patients to identify those that carry a drug resistant bacteria

• monitoring levels on drug resistant infections

prescription of antibiotics

only prescribed when the patient is definitely suffering from a bacterial infection and the antibiotic must be targetted to the specific bacterium. patients are educated and advised to complete their full course of antibiotics so that all the bacteria are destroyed. different antibiotics should be used for repeat infections

body temperature after death

immediately after death the body starts to cooldue to absence of heat producing chemical reactions, eg respiration

sigmoid curve

cooling follows a sigmoid curve where body temperature is only useful for 24 hours, assuming that the body was at normal temperature (37^o) at death. that might not be the case if the person is suffering from fever or hypothermia

factors to consider when using body temp to determine time of death

• ambient (environmental) temperature

• body size

• body position

• status of clothing

• air movement

• humidity

• whether the body was immersed in water

• general surroundings eg inside or out

rigor mortis series of events

• after death, muscle cells become starved of oxygen and aerobic respiration stops

• respiration becomes anaerobic and produces lactic acid

• pH of the cells fall, inhibiting enzyme activity which stops anaerobic respiration

• ATP is no longer produced which prevents the bonds between muscle proteins from breaking

• proteins can no longer slide over each other to shorten or relax the muscle which fixes muscle and joints in position

smaller muscles stiffen and pass through rigour first

increased environmental temperature or physical activity before death can bring on rigour faster

rigor mortis starts to pass as muscle tissue starts to decompose, and is only useful for determining time of death for 3-48 hours after death

autolysis

immediately after death digestive enzymes from the digestive tract and lysosomes start to break down cells

role of bacteria in the first stages of decomposition

bacteria from the digestive tract and gas exchange system rapidly invade the tisses and release digestive enzymes (putrefaction). the lack of oxygen promotes the growth of anaerobic bacteria

first signs of decomposition

a greenish discolouration appears on the skin of the abdomen due to anaerobic bacteria converting haemoglobin into sulfahaemoglobin. the discolouration spreads across the body darkening to reddish green then purple black

time frame starts between 36-72 hours

during and after gas phase

bacteria produce gases in the tissues and intestines and the body becomes bloated and starts to smell. after more decomposition the gas escapes and the body deflates and the fluid drains away. the tissues shrink and the body starts to dry

time frame usually after 7 days

factors that affect the rate of decomposition

• environmental temperature- low temps slow, warm speeds up

• clothing- insulation

• body injuries- aid entry of bacteria

• intense heat- slows as enzymes denature

entomology

the study of insects and their life cycles

details collected from body after death

• samples of each insect and where each was found

• air body and ground temperature

• maggot mass

• information about location of the body

• condition of body and temperature history of location

use of maggots in forensics

length of maggot mass is measured to determine age which can be compared to a growth graph to estblish their age- fly life cycle timings can help determine when the egg was laid. only reliable if ambient temp was constant

live ones are fed and allowed to grow to identify species

no way of knowing when the fly found the body

factors that can affect estimation of time of death

temperature affects rate of progress through life cycle and growth

toxins in the body can speed up or slow down maggot and pupal development

factors that determine species of insect present at body

stage of decomposition, presence of other insects

colonisation of insects as a form of succession

one group of organisms feed on the body to change the conditionsso they becomes attractive to another group

these are replaced by other species over time- species richness increases

DNA profiling

genetic/DNA fingerprinting that relies on ever individual’s DNA being unique

STR (short tandem repeats)

found within introns - short DNA sequences repeated many times and can contain 2 to 50 base pairs and can be repeated from 5 to several hundred times. the same STR sequence occurs at the same locus aton both homologous chromosomes but the number of times it is repeated differs

STR uses in DNA profiling

each person has a large number of introns with lots of STR loci as there is a large amount of variation in the number of repeats for each STR sequence it is unlikely that two individuals will have the same number of repeats and the profile wil be unique for each

possible sources of DNA for forensic purposes

any biological tissue- cheek swab cells, wbcs, bone marrow in a skeleton, sperm, skin, hair, saliva, fingerprints

how DNA is extracted from cells

the cell membrane is disrupted in a buffer solution. DNA is seperated by centrifugation and the sample is incubated with proteases. the DNA is precipitated out and washed in ice cold ethanol

identification of where to cut DNA

cut at a specific recognition site- specific base sequence which are palindromic when double stranded and cut to produce a sticky end or a blunt end. each restriction enzyme will only ever produce a sticky or a blunt end not both

restriction enzymes in DNA profiling

if the same restriction enzyme is used to cut the same DNA sample or two DNA samples from the same individual the fragments produced will be identical in every case

PCR (polymerase chain reaction)

technique used to copy STR sequences

Gel electrophoresis

technique used to seperate the STR sequence fragments

techniques used to visualise the fragments

• southern blotting- STR DNA fragments are transferred to nylon membrane which is then incubated with a DNA probe with complementary base pairs to the fragments (hybridisation). probe may be radioactive or flourescently tagged

• flourescent orimers in PCR remain attatched to the DNA after the last cycle and pass the gel to be detected and analysed by a computer

possible uses of DNA profiling

identifying criminals and victims, settling paternity disputes, identifying stolen animals, confirming pedigree lineage, identifying evolutionary relationships and species

conclusion reaching from a DNA profile

each STR fragment shows up as a band on the gel- comparing the position of the bands from test sample and reference sample confirms a match

why is DNA profiling not infalliable

the profiling procedure has many steps where cross contamination would occur

only some STR sequences are analysed

there is a chance another individual has an identical profile