Principle of infection 1

Question 1

Microbiota

what is this?

Answer 1

The human microbiota consists of the 10-100 trillion symbiotic microbial cells harboured by each person. We have about 10 times as many microbial cells as human cells.

Question 2

Microbiome

what is the collective?
where?

Answer 2

The microbiome is defined as the collective genomes of the microbes (composed of bacteria, bacteriophage, fungi, protozoa and viruses) that live inside and on the human body.

Question 3

Commensalism

what do they do? what is special about them?

Answer 3

Commensals are microbes that derive food and shelter from the host. They normally do no harm to the host. The host may benefit from the presence of the microbe. This relationship is described as mutualism. Commensals are also sometimes referred to as the “normal flora.”

Question 4

normal flora examples

skin?
oral cavity?
large bowel?
nose?
nasopharyn + upper resp tract?
vagina?

Answer 4

Skin surfaces - staphylococci, corynebacteria and streptococci

Oral cavity - streptococci, staphylococci, corynebacteria, anaerobic organisms, yeast (fungi)

Large bowel - anaerobic bacteria e.g. Bacteroides sp., a number of coliforms e.g. Escherichia coli, and faecal streptococci e.g. Enterococcus sp.

Nose - staphylococci, streptococci, corynebacteria

Nasopharynx & upper resp. tract - streptococci, staphylococci, corynebacteria, Neisseria sp.

Vagina - Lactobacillus sp, Bacteroides sp, Enterococcus sp.

Question 5

Difference between commensal and infection

what is an infection?

Answer 5

Commensal microbes (normal flora) are said to colonise the host. No harm is done to the host. It is important to distinguish colonisation from infection. Infection implies that harm is done to the host i.e. causes disease

Question 6

pathogen
what is it?
how does body react to this?

Answer 6

A microbe that causes disease is a pathogen. Usually the host will manifest an inflammatory response to a pathogen, but not to a coloniser (commensal) at a normally non-sterile site of the body.

Question 7

Commensals acting as pathogens - opportunistic infection

what is opprtunistic infection?
example of this?
what will happen in healthy individual?

Answer 7

If the host’s defences are weakened (immunocompromised) infection may occur; this is called opportunistic infection.

An example of opportunistic infection is a patient infected with an organism that may be found normally in the environment and which normally causes no harm to a healthy individual. In patients with advanced HIV infection and with very depressed immunity (AIDS), lung infection may be seen with Pneumocystic jiroveci (a fungus of low virulence).

In normal healthy people exposed to Pneumocystis jiroveci no infection occurs as the intact immune system will prevent this

Question 8

Virulence

Answer 8

describes the degree of pathogenicity

Question 9

IF we move commesal colony to another area

some examples of this

Answer 9

Some organisms that are commensals at one body site may be pathogens at another body site e.g. Staphylococcus aureus in the nose (commensal) but Staphylococcus aureus in a post-operative wound infection (pathogen) or Escherichia coli in GI tract (commensal) but Escherichia coli in urinary tract causing UTI (pathogen).

Question 10

if sterile areas get infected

examples of sterile sites?
what happens if organism in sterile site?

Answer 10

Some body sites do not have a normal flora. These sites are sterile and include lower respiratory tract, blood, bone, joint and subcutaneous connective tissue, female upper genital tract, urinary tract, CNS including CSF and eye, and other viscera e.g. liver, spleen and pancreas.

If any organism gains entry to a normally sterile site, an infection may occur. Staphylococcus epidermidis on skin (commensal) but Staphylococcus epidermidis bloodstream infection following infection of intravenous line (pathogen). The organism has been “helped” to bypass the normal body defences i.e. the skin.

Question 11

Human Microbiome Project challenging sterility idea

why is findings unclear? (2)

Answer 11

some organs traditionally though to have no normal flora, do in fact have a microbiota. The significance of these findings is still unclear as many of these organism are not cultureable and can only be detected by molecular methods

Question 12

pathogen, harm or good?

what are 2 kinds of organisms?
give examples

Answer 12

Some organisms are strict pathogens i.e. will always cause disease e.g. Bacillus anthracis (anthrax). Highly pathogenic microbes are said to be virulent.

Some pathogens may sometimes almost behave like commensals, though they would not be considered to be part of the normal flora e.g. Salmonella typhi (typhoid) (carrier state).

Question 13

Balance between microbe and host

what if balance tips?

Answer 13

if balance in favour of microbe, infection occurs

if balance in favour of host, no infection occurs

Question 14

Steps Required for Establishment of Infection

6 steps?

Answer 14

1. Survival of microbe and spread to the host
2. Adherence of microbe to the host and entry into the host
3. Multiplication
4. Evasion of host defences
5. Damage to the host
6. Shedding of microbe and spread to the environment or to another host

Question 15

saprophytes

what are these and how do they survive?

Answer 15

Some microbes can survive as saprophytes, these are harmless microbes that live on inanimate material and derive their nutrition from environmental sources.

Question 16

Survival of microbe

Describe where microbes can survive
describe two ways microbes may survive in harsh environments (2)
how may they outcompete other microbes?

Answer 16

Microbes must be able to survive in the environment in which they live, some can only survive in the environment provided by the host while others can survive in air, water etc.

Some organsisms produce spores which can survive heat and dryness, others have a waxy coat. There may also be competition with other microbes so they release bacterocins to kill these off.

Question 17

Routes of spread

5 routes? examples?

Answer 17

Airborne/droplets (called aerosol) = respiratory infection
GI tract/ Faeco-oral route
Skin (e.g. plasmodium = malaria)
Through inanimate objects (called fomites) (e.g. RSV)
Other epithelial surfaces (e.g. STD’s)

Question 18

Adherence and Entry into Host

how may they be able to do this? (2)

Answer 18

In order to gain a foothold in the host, a microbe needs to adhere to the hosts tissue. This is done by various mechanisms including attachment to tissue via fimbriae (pili), adhesins and other molecules present on the surface of some of these microbes. E.g. attaching to urethral epithelium

Question 19

Multiplication

where will they multiply?
what do they procude during mutliplication?

what may happen following multiplication? (2) examples?

Answer 19

Once microbes are internalised, either by an active process as in many virus infections or introduced passively as most bacterial infections, they will multiply locally to produce a focus of infection.

During the process of multiplication many microbes produce various tissue destroying enzymes which allow invasion of surrounding tissues as well as deeper sites. Some bacteria produce toxins and tissue destroying enzymes, these toxins are capable of local or systemic actions.

Following multiplication the infection may remain localised, e.g. abscess or spread to contiguous tissues e,g, cellulitis or spread via the blood – septicaemia.

Question 20

Evasion of Host’s defences

Answer 20

The hosts defence mechanisms soon mobilise to contain the infection, microbes use a variety of methods to evade the hosts response.

As mentioned earlier a successful pathogen will tip the balance in their favour in comparison to hosts and evade the hosts defences. Damage to the host may result from effects of the microbe or sometimes from the immune response of the host

Question 21

Shedding Infection

how do they exit host? (5)
examples?

Answer 21

In order to perpetuate the infection, the microbe must find a new host. To do this it must exit the host and it may do this by: respiratory tract (airborne e.g. TB), GI tract (e.g. cholera), genital tract (e.g. gonorrhea), blood sucking insects (e.g. malaria), from hands, saliva to fomite

Question 22

Symptoms of Infection (6)
Local symptoms

what does pus mean?

Answer 22

Local symptoms (inflammation): Redness, swelling, warmth, pain, loss of function, pus (=pyogenic infection)

Question 23

Symptoms of Infection (5)

Systemic symptom

Answer 23

Systemic symptoms = Fever, rigors, chills, tachycardia, tachypnoea

Question 24

Symptoms of Infection
Acute infections

how fast does it progress?
what kind of symptoms?
examples?

Answer 24

Acute infections: Infection established and disease progresses rapidly with major local and systemic manifestations. E.g. pyogenic skin lesion or toxin disease.

Question 25

Symptoms of Infection
Chronic infections

speed of progress?
why does this infection take place?
example?

Answer 25

Chronic infections: Usually slower onset, but may still have major local and systemic symptoms, a chronic inflammatory response results when the host does not succumb immediately to the infection but cannot clear it e.g. TB

Question 26

Symptoms of Infection
Asymptomatic infection

what does this mean?

Answer 26

Asymptomatic infection: Infection with a pathogenic microbe and the inflammatory response is mild or none at all and damage to the host is mild or none at all. No symptoms present.

Question 27

chemical barriers are generally the constituents of Innate Immunity and their characteristics

5 key characteristics?
1 - specificity?
2 - repeat exposure effect?
3 - special environments?
4 - 3 chemical substances that have an inhibitory effect?
5 - examples of phagocytic cells? (3)

Answer 27

a) Immunity is non-specific i.e. are active against a variety of infectious agents.
b) Non-specific immunity can not be enhanced by repeat exposure.
c) Microbicidal substances e.g. lysozyme in tears, gastric acid and antibacterial substances in urine.
d) Acute phase proteins, interferons and complement are chemical substances and are all associated with mild to moderate inhibitory effect against various microbes.
e) Phagocytic cells e.g. Mononuclear phagocytes-monocytes and macrophages, and natural killer (NK) cells.