Infectious Agents Flashcards
List some differences between prokaryotic and eukaryotic organisms.
EUKARYOTIC:
- contains a chromosome
- has a nucleus
PROKARYOTIC:
- no nucleus
- no mitochondria
- mainly circular DNA (plasmids)
4 TYPES OF PATHOGENS
- Viruses
- Bacteria
- Fungi
- Parasites
Describe the structure of a virus.
- DNA or RNA genome is linear, segmented, single or double stranded
- has a protein nucleocapsid (individual capsomeres in a helical, cubic or complex arrangement)
- no cytoplasm
- it may be enveloped or non-enveloped (may have components derived from the host cell) - meaning may or may not have outer membrane
- it is an obligate intracellular organism (meaning it can only replicate inside host cells)
- they have receptors on the nucleocapsid that allow it to bind to cells
- 20nm - 300nm
VIRAL LIFE CYCLES include
- Recognition
- Attachment
- Penetration
- Fusion
- Uncoating
- Transcription
- Protein synthesis
- Replication
- Envelopment
- Budding and release
- Lysis and release
EXAMPLES
- Herpes Virus - enveloped, icosahedral nucleocapsid
- HIV
- Ebola Virus
- Flu Virus e.g influenza - segmented RNA genome, surface has haemagglutinin, enveloped and pleomorphic
- Norovirus
- Adenovirus - non-enveloped, has a DNA genome
Describe the structure of bacteria.
- 0.5 μm to 3 μm
- prokaryotic
- contains haploid DNA as a circular genome
- has no nucleus
- usually have a rigid cell wall outside the cytoplasmic membrane (which retains the osmotic potential)
- has no mitochondria
- divides by binary fission
- some can ferment lactate
- have pili and fimbriae allowing bacteria to attach to surfaces
- some have flagella - allow bacteria to be motile
CLASSIFIED USING
- rRNA sequencing
- Genome sequencing
- MLST or RFLP typing – genetic variation typing methods
MORPHOLOGY
- Structurally appear as cocci, rod/bacilli, coccobacilli, spirochetes
- Atypical bacteria e.g. mycoplasma (no cell wall), chlamydia (biphasic)
EXAMPLES
- Campylobacter jejuni - causes food poisoning
- Clostridium perfringens - Gas Gangrene
- MRSA - antibiotic resistance
- Meningococcus B and C have serotype groups
BINARY FISSION
- Genome replicates
- genes move to either side of bacteria
- septum forms down the middle of the bacteria
- bacteria splits
How does the Gram stain help us identify different bacteria?
The Gram stain divides bacteria into:
GRAM-POSITIVE - blue/purple
GRAM-NEGATIVE - red/pink (when using safranin)
These bacteria have differing cell wall structures which accounts for the different staining properties.
The presence of beta-hemolytic colonies indicates the possibility of Streptococcus pyogenes infection
Describe bacterial growth and list some of its requirements.
Bacterial growth occurs through binary fission. Their doubling time is approximately 20-30 minutes.
Their growth requires:
- carbon, water and nitrogen
- nutrients and minerals
- the correct pH
- the correct osmotic pressure
- the correct temperature
- the correct atmosphere
Describe the different classifications of bacteria based on their growth in different atmospheres (ie. O2).
There are two classes: Aerobes and Anaerobes.
Of the aerobes, we have:
- Strict aerobes: which will grow only in O2
- Facultative aerobes: which can grow in both atmospheres
Of the anaerobes, we have:
- Strict (obligate) anaerobes: which will grow only in the absence of oxygen
- Microaerophilic anaerobes: which grow best with small traces of oxygen
Describe the structure of fungi (mycoses).
- eukaryotic
- have haploid or diploid DNA
- have single or multiple nuclei
- have a rigid CHITINOUS cell wall outside the cytoplasm
- they can be single or multicellular
- over 2 μm
They come in two forms:
YEAST: spheroid single cells budding and dividing
FILAMENTOUS FORM/ MOULDS: multinucleate branching hyphae forming mycelium
Classification is complex and is based on morphology (growth form), mode of reproduction (sexual or asexual) and disease caused: 70,0000species ~300 human pathogens
EXAMPLES
- Tinea pedis - athlete’s foot - Dermatophytes , multiple septate branching hyphae on microscopy
-Cryptococcus neoformans - fungal yeast like form – encapsulated – large capsule
Oral Candidiasis / Candidosis
- most common oral fungal infection
- carriage rates 20% - 75% without any symptoms.
- Usually Candida albicans
What is the simplest way to classify fungi?
The classification of fungi is complex, and is based on morphology (growth form), mode of reproduction (sexual or asexual) and the disease caused.
The easiest way to look at different fungi is to consider the type of infection they cause.
- SUPERFICIAL mycoses: affect the skin, hair, nails and mucous membranes E.G THRUSH
- CUTANEOUS mycoses: affect the deep skin E.G RINGWORM
- SYSTEMIC mycoses: affect the body’s systems E.G HISTOPLASMOSIS
Describe the structure of parasites (protozoa)
- they are single cells (5 to 300 μm)
- they have single or multiple nuclei
- they have haploid DNA
- their morphology varies throughout the life cycle
- they may have flagella
- trophozoites, cysts and other morphotypes are parts of their complex life cycles
- eukaryotic
- Plasmodium falciparum - malaria
- Enterobious virmicularis - a nematode – common in UK children, threadworm or pinworm, asymmetical shape (50 uM), sticky tape mount to pinworm eggs
- Trichomonas vaginalis - common STI - structured as trophozoites
Describe helminths (a type of parasite).
Helminths are multicellular parasitic worms. They have a different morphology and sizes at different life cycle stages. They can either be microscopic or visible to the eye.
They are categorised into three groups:
CESTODES are tapeworms - Egg shed in faeces (human), cysts in muscle
Adult tapeworm (human)
- 2 – 4 metres long
- 700- 1000 segments
- TREMATODES are flatworms or flukes
e.g Schistosoma haematobium
Egg shed in urine (human) (via snails)
Metacercaria in water (burrow into skin)
Adult fluke in human
- NEMATODES are roundworms
e.g Ascaris lumbricoides
Egg shed in faeces
Adult worm
How are microbes visualised?
Bacteria, fungi, protozoa and certain lifecycle stages of helminths can be visualised using light microscopy.
BACTERIA
- Unstained at x400
- Motile bacteria with flagella move rapidly. Unmotile also move using Brownian motion.
Viruses at too small to be seen using light microscopy but can be visualised with an electron microscope after special staining. This allows detailed morphologic examination of the viruses. MAGNIFICATION is usually x100,000
Outline the two methods by which organisms are classified and named.
FIVE KINGDOMS
Protista
Plantae
Animalia
Fungi
Monera
THREE DOMAINS - discovered by DNA analysis
Eukarya
Bacteria
Archaea
CRITERIA THAT CLASSIFICATION IS BASED ON
- Structural and functional and metabolic observations
- Molecular and genetic and genome analysis
Classification is needed to understand
- Biology and microbiology
- Evolution and phylogenetics
- Pathogenesis of infectious diseases
- Life cycles
- Accurate diagnosis
- Effective treatment
- Public health control of infectious diseases
LEVELS OF HIERARCHY
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
Strain
Isolate - the particular pathogen from the particular person
Write the species name In italics. The first letter of the first word is capitalised and the first letter of the second word is always lowercase
