Infection and DiseaseFungi

ParasitesNosocomial infection

Diagnosis of infectious disease

Fungi

Fungi

• Eukaryotes – nucleus / several organelles

• Non-motile

• Aerobic

• Cell wall made of Chitin

Need for special antibiotics (antifungals):

Fungal diseases –superficial infections

• Superficial infection– ability to infect intact skin /nail

• Caused by dermatophytes

• These fungi use keratin for their growth

• Eg. Trychophyton spp

Fungal diseases –Systemic infections

• Systemic infections:

• Lung infections in healthy individuals

• Not common Eg. Histoplasma spp

Fungi opportunistic infection

• Infection in immunocompromised individuals

• Eg. Aspergillus spp. and Candida spp. in AIDS patients

• Can affect any organ

Fungal toxins

• 1960s – “Turkey X disease” – Brazilian peanut feed

• Linked to fungal toxin from Aspergillus spp. (Aflatoxin)

• Can affect liver, kidney , heart and brain in humans

Parasites• Unicellular (Protozoa) – eg.

Amoebae

• Multicellular worms (Helminths) eg. Round worms /flatworms

• Many parasites require – 2 hosts to complete their life cycle (eg. Plasmodium falciparum)

• Some parasites require vectors for transmission(eg. Mosquitoes)

Entamoeba spp

Round worms

Protozoa

• Eg. Entamoeba spp. – amoeba that can cause dysentery / bloody diarrhoea

• Plasmodium falciparum – causes malaria

Helminths

Nematodes

• Unsegmented worms

• Roundworms

Platyhelminthes

• Segmented worms

• Flat worms

Roundworm (Ascaris spp.) - life cycle

< 100µm

500µm

40 cms

Abdominal painDiarrhoea

Tapeworm (Taenia spp. ) – life cycle

< 50µm

5mm

Up to 10 meters

Indigestion, loss of appetite, abdominal pain

Nosocomial infections or hospital acquired infections

Nosocomial or hospital acquired infections infections

Hospital acquired infections: new infections acquired in the hospital or immediately after discharge

Example of nosocomial infections

MRSA – Methicillin Resistant Staphylococcus aureus ; highly antibiotic resistant

Diagnosis of infectious diseases

Methods in Diagnosis of infectious diseases

Direct methods:• used to detect the microbe

or its components

Indirect methods: • used to detect antibody

response in the host to a microbe

• Direct methods - Microscopy - Culture - Antigen detection - Detection of genomes

• Indirect methods - Serology (Ab detection)

Choosing the right sample

• Choosing the appropriate sample is critical

Factors:• Type of illness• Time from appearance of

symptoms

Typhoid feverWeek 1-2 – bloodWeek 3 - StoolWeek 4 - Urine

Window period

• Period between infection and the point when the lab test can identify infection

• Varies with the test used. (Shorter for direct tests than indirect tests)

Choosing the right test

Dengue• 1-6 days – NS1 antigen

• 5-15 days – IgM Ab against dengue virus

Direct methods

(1) Microscopy

• Light microscopy used for visualization of bacteria, fungi, parasites

• Staining – enhances contrast

• Good for rapid presumptive Dx

Bacteria - unstained

Bacteria stained

Microscopy

• Quick, easy, inexpensive

• Good for microbes that are difficult to grow – MTB, malarial parasite

• Viruses – require electron microscopy

Bacteria in urine

MTB in sputum

Malarial parasite in blood

(2) Culture• Good when the number of microbes in

the sample is low

• Culture media -used for bacteria and fungi

• Human cell lines – used for some viruses

• Time – 2 days to 8 weeks

• Expensive

• Subsequent tests are required for specific identification

Bacteria

Poliovirus in human cells

Fungi

(3) Antigen detection of microbial antigens

• Detection of microbial proteins (antigens) allows specific identification of microbe

• Easy, inexpensive and reliable methods available

Antigen detection: Immunofluorescence

Specific Ab tagged with a fluorochromeRequires fluorescence microscope

Antigen detection: ELISA

Anti-Target Antibody

Enzyme linked immunosorbent assay = ELISA

ELISA Principle

Anti-Target Antibody

Target Protein

ELISA Principle

Anti-Target Antibody

Target Protein

Antibody-Enzyme

Conjugate

Anti-Target Antibody

Target Protein

Antibody-Enzyme

Conjugate

Substrate

ELISA Principle

ELISA-Antigen detection video

https://www.youtube.com/watch?v=70TPrfL_8-M

(4)Detection of microbial genomes

Agarose gel electrophoresis

Applications of PCR

• Highly sensitive (1-10 copies are picked up)

• Early Dx

• Can quantitate microbes - virus loads / response to treatment

• Works on non-cultivable microbes

Indirect methods

(1)Detection of antibodies produced against microbes

Ab detection ELISA Eg. HIV Ab detection ELISA

Ab in sample

Point-of-care tests

Point-of-care tests

• Tests performed at the point-of-care (eg. doctor’s office)

• Can be performed outside a laboratory

• Does not require special training

Point-of-care tests for Ag and Ab detection

• Principle –similar to ELISA

• Rapid (< 10 minutes)

• Available for many infectious agents (eg. HIV, rotavirus..)

Antigen Antibody