Microorganisms & Disease

How do microorganisms cause disease?

Microorganisms cause disease through direct injury to host cells; toxins or enzymes that damage cells; and damage caused by the host immune response

What is tissue tropism and host range?

Tissue tropism is the specific cells or tissues a microorganism can infect based on receptor binding; host range is the range of species it can infect

What are endotoxins and exotoxins?

Endotoxin is LPS found in the outer membrane of Gram-negative bacteria and can cause inflammation fever and septic shock; exotoxins are specific toxins secreted by microorganisms that cause particular effects

How can the immune response cause disease?

The immune response can damage tissue through destruction of infected cells excessive immune responses and chronic inflammation; for example tuberculosis can cause lung damage through granuloma formation

Why was Spanish influenza so severe?

It involved a new haemagglutinin subtype deep lung infection suppression of antiviral responses increased inflammation and secondary bacterial infections

How do innate and adaptive immunity differ?

Innate immunity is rapid non-specific and has no memory; adaptive immunity is slower highly specific and produces memory

What are the main components of innate immunity?

Phagocytes engulf microorganisms; complement helps destroy bacteria; antigen-presenting cells present pathogen antigens to other immune cells

What happens during an adaptive immune response?

Antigens are recognised → immune cells proliferate and differentiate → antibodies are produced → infected cells are destroyed → memory cells remain

What are B cells and CD8 cells?

B cells produce antibodies and provide humoral immunity; CD8 cells destroy infected cells through apoptosis

What is the difference between primary and secondary immune responses?

The primary response is slower and weaker after first exposure; the secondary response is faster and stronger because of memory cells

How do pathogens evade the immune system?

They can hide inside cells; change their antigens through antigenic drift or shift; or directly inhibit immune responses

How is infectious disease diagnosed?

Diagnosis combines signs and symptoms with geographic location incubation period and possible exposure; diagnostic testing is usually required to confirm infection

What is the difference between ELISA and PCR?

ELISA detects proteins or antibodies and can be affected by cross-reactivity; PCR detects pathogen DNA or RNA and provides strong evidence that genetic material is present

What is sensitivity vs specificity?

Sensitivity measures how well a test identifies people who have the disease and high sensitivity means few false negatives; specificity measures how well it identifies people without the disease and high specificity means few false positives

What is direct vs indirect diagnosis?

Direct tests detect the pathogen or its components such as microscopy antigen detection PCR and culture; indirect tests detect the host immune response such as antibodies through serology

What are the main diagnostic methods?

Microscopy directly observes pathogens; staining identifies microbial characteristics; culture grows pathogens; serology/ELISA detects antibodies or antigens; PCR detects DNA/RNA; mass spectrometry identifies microbial characteristics

What are the advantages and problems of culture?

Culture can identify pathogens by growing them on specialised media but can take days to weeks; some pathogens are difficult to grow and slow diagnosis can allow transmission to continue

How do antibacterials antifungals and antivirals work?

Antibacterials target bacterial structures such as cell walls ribosomes and membranes; antifungals target fungal structures but are harder to develop because fungi are similar to human cells; antivirals inhibit viral replication

What is antimicrobial resistance?

Microorganisms can develop genetic changes that reduce the effectiveness of antimicrobial drugs; resistance genes can also spread between different groups of bacteria

How does vaccination prevent disease?

Vaccines expose the immune system to antigenic material such as microbial components proteins or toxins/toxoids; this creates adaptive immune memory and protection against future infection

What are other methods of preventing infectious disease?

Treatment can reduce microbial load; prophylactic drugs can prevent infection before exposure; behaviour modification reduces transmission; vector control uses methods such as bed nets repellents and reducing mosquito populations

What are the key facts about smallpox?

Smallpox is caused by Variola virus and spreads through infectious droplets; humans are the only host; historical mortality was around 20–30%; global vaccination led to eradication

What are the key facts about HIV?

HIV targets and progressively damages the immune system; it has a long incubation period and can be infectious before severe symptoms; transmission occurs through infected bodily fluids such as blood semen and breast milk

What are the key facts about malaria?

Malaria is caused by Plasmodium and transmitted by Anopheles mosquitoes; transmission is influenced by temperature rainfall humidity and mosquito populations; Plasmodium can change its surface antigens allowing repeated infections

What are the key facts about influenza?

Influenza infects humans pigs and birds; seasonal influenza changes mainly through genetic drift; pandemic influenza can arise through genetic reassortment or antigenic shift when different strains infect the same host

What are the key facts about cholera?

Cholera is caused by Vibrio cholerae and is mainly transmitted through the faecal-oral route via contaminated water; it causes acute diarrhoeal disease and outbreaks are associated with poor sanitation flooding and disrupted water supplies

What are the key facts about Ebola?

Ebola causes haemorrhagic fever and is mainly spread through infected bodily fluids and close contact; limited global spread is partly due to high mortality limited host range and lack of normal airborne transmission; ring vaccination can be used during outbreaks

What are the key facts about plague?

Plague is caused by Yersinia pestis and is commonly transmitted by infected fleas carried by rodents; bubonic plague causes buboes; septicaemic plague spreads through the bloodstream; pneumonic plague infects the lungs and can spread through respiratory droplets

What makes a pathogen capable of causing a pandemic?

High transmissibility; suitable transmission route and host range; ability to infect susceptible cells; immune evasion; appropriate incubation/infectious period; genetic variation; ability to spread before detection; and ability to cause disease without killing the host too quickly

Why doesn't a pathogen necessarily benefit from killing its host quickly?

If the host dies very quickly the pathogen has less opportunity to spread