Infection and Disease Process

What are bacteria?

Answer: unicellular organisms with a rigid cell wall (peptidoglycan), no organised intracellular organelles and a single chromosome of DNA. Some can have structures on the cell surface, such as a flagella (tail-like structure), which enables them to attach to host or aid in motility. Bacteria reproduce by simple division due to the single chromosome of DNA.


Bacteria are classified by their shape, oxygen requirements and Gram staining:


Shape:

• Cocci, meaning they are round
• Bacilli, meaning they are rod-shaped
• Fusiform, meaning they are spindle-shaped
• Spirochetes, meaning they are tightly spiralled

Oxygen Requirements:

• Anaerobic (oxygen is toxic to them)
• Aerobic (require oxygen to survive)

Gram Staining:

• Purple, meaning there is a lot of peptidoglycan in the cell wall
• Pink, meaning there is only 10% peptidoglycan in the cell wall

What are viruses?

Answer: small infectious agents (smaller than bacteria) and require living hosts which they invade.


The way in which viruses function is very different to bacteria. They only survive once they've invading a living host cell, and then use that host cell's DNA to produce new viruses. The new viruses stimulate cell lysis, permitting the new viruses to be released from the original host cell and then 'infect' others. Viruses have no organised cellular structure with either DNA or RNA which mutates frequently and alters their genome, which is why viruses are so hard to treat.


The sum of events of virus action is outlined below:

1. Penetration of host cell via endocytosis or fusion with the cell membrane
2. Insertion of the viral genome into the host cell genome
3. Host cell sheds viral replicants which infect other cells by budding the enveloped viruses or lysing to release them into the interstitial fluid
4. Viruses can remain latent inside the host cells, activated at later times of stress or decrease in immunity
5. Can be associated with causing cancer, e.g. papillomavirus

What are fungi?

Answer: free living eukaryotes that are multicellular and have a complex metabolism.


Unlike bacteria and viruses, very few fungi are capable of producing disease in humans, most being incidental, self-limiting infections of skin/subcutaneous tissue, for example Tinea and athlete's foot. However, fungi-caused diseases can be life-threatening in very ill/immunosuppressed patients.

What are parasites?

There are three types of parasites: protozoa, arthropods and helminths (worms). Not in Australia but in developing countries where hygiene is poor, parasites are a major concern. This is because parasites infect and cause disease in animals, and the infection is then transmitted to humans via:

• Cysts or spores in animal faeces (contaminated water or food)
• Arthropod vectors, e.g. malaria via mosquitos
• Close contact, e.g. lice

Infection

Infection is when pathogenic microorganisms are able to invade the host, attach and multiply in the tissues. It refers to the direct functioning of the above modes of infection.


Infectious disease is when a host organ sustains injury or pathologic damage in response to an infection, i.e. the infection in a local area has caused long-term damage to tissues.


Sources of infection include:

• Exogenous, meaning an external source
• Endogenous, meaning an internal source
• Another human
• Animals, e.g. rabies
• Nosocomial, meaning hospital acquired
• Community acquired

There are several ways by which infection spreads, which are:

• Local
• Lymphatic
• Haematogenous
• Tissue fluids
• Neural spread (along nerves)

There are stages to infectious disease, like there are to many other conditions:

1. Incubation period with no symptoms
2. Predromal period where there are nonspecific symptoms
3. Illness with typical symptoms of the particular infection
4. Convalescence, where the symptoms fade
5. Recovery or chronic infection/carrier state

However, not all patients make a recovery. During the illness phase (3.), the infection can worsen to septicaemia, i.e. blood poisoning, in which the infection moves into the bloodstream. This leads to overwhelming infection and mostly death.

What are normal flora?

Answer: microorganisms that establish permanent residence in or on the body without causing disease, and acquire nutritional needs and shelter from the host. They are found on the skin and mucous membranes of all body openings. They are not, however, present in blood, lymph, spinal fluid or most internal organs.


Normal flora are of high importance, e.g. the bacteria in the bowel extract nutrients form the host and secrete essential vitamin by-products of metabolism. Normal flora also:

• Stimulate antibody production
• Produce antibiotics
• Prevent entry of pathogens by occupying a space
• Maintain pH

NOTE: Taking a course of antibiotics kills normal flora and therefore there will be more nutrients available to other infectious agents, increasing their growth and causing thrush.


Opportunists are pathogens capable of producing disease when the health/immunity of the host is weakened (through illness or medical therapy). Unfortunately this includes normal flora and microorganisms that do not normally cause infectious disease, e.g thrush. This is common to patients with HIV/AIDS.