Infectious agents

Question 1

Classification of Microbes

what are the 4 groups?

Answer 1

4 groups

bacteria
fungi
virus
parasites - Protozoa and helminths

Question 2

Viruses

size?
structure? - dna/rna? contained where? what arrangement?
cytoplasm?
what is special about herpes simplex virus?
intra or extracellular?

Answer 2

Size: 20 nm to 300 nm

Structure
DNA or RNA (not both) core within protein capsid in a helical, cubic or more complex arrangement.
There is no cytoplasm.
Some viruses may have an envelope derived from host cell e.g. Herpes simplex virus.
All viruses are obligate intracellular organisms

Question 3

example of viral infections

upper resp tract examples
GiT example
hospital acquired examples

Answer 3

Upper respiratory tract infections caused by e.g. rhinovirus, influenza, respiratory syncytial virus (RSV)

Gastroenteritis causing viruses e.g. norovirus (SRSV), rotavirus and adenovirus

Hospital-acquired infections include gastroenteritis caused by norovirus

Question 4

Bacteria

size?
structure? - Dna? nucleus? mitochondria? cell wall?

Answer 4

Size: 0.5 μm to 3 μm

Structure 
Prokaryotic 
Haploid DNA and no nucleus 
They usually have a rigid cell wall outside the cytoplasmic membrane 
There are no mitochondria

Question 5

example of bacterial infections

UTI caused by?
another example? what causes it?

hospital acquried infections?

Answer 5

Urinary tract infections (UTI) caused by Escherichia coli and pharyngitis caused by Streptococcus pyogenes.

Healthcare-associated infections (Hospital-acquired) infections include post-operative wound infection caused by Staphylococcus aureus, especially methicillin-resistant strains i.e. MRSA, and ventilator-associated pneumonia (on ITU) caused by Pseudomonas aeruginosa

Question 6

Fungi

size?
structure? pro or euk? DNA? nucleus? cell wall? multi or uni cellula?

what is classification based on?

Answer 6

Size: over 2 μm

Structure:
Eukaryotic with haploid or diploid DNA.
They may have single or multiple nuclei.
There is often a rigid chitinous cell wall outside the cytoplasm.
They may be multicellular e.g. mushrooms or single cell e.g. yeast or filamentous form/moulds.
Some may be dimorphic i.e. they can exist in either of the two forms.
The classification is complex and is based on morphology and mode of reproduction e.g. whether it is sexual (teleomorph) or asexual (anamorph).

Question 7

example of fungal infections

what are they classified by?
examples?

Answer 7

easier to classify fungi by the type of infection they cause e.g. Superficial mycoses of skin, nails, hair

mucous membranes e.g. ringworm (dermatophytes) or thrush (Candida albicans), or Subcutaneous mycoses e.g. mycetoma, or Systemic mycoses e.g. Histoplasmosis.

Question 8

Parasites

2 different parasites?

Answer 8

parasite is sometimes applied to any microbe that lives in or on another animal host, causing it harm

But is the 4th group too

Question 9

Protozoa

cells?
size?
nucleus?
DNA?

Answer 9

Single cell. 
5 to 300 μm. 
Single or multiple nuclei. 
Haploid DNA. 
Morphology varies throughout life cycle. May have flagella.

Question 10

Protozoa - examples and what they cause?

Answer 10

Plasmodium sp. which causes malaria

Entamoeba histolytica which causes amoebic dysentery

Question 11

Helminths

cells?
size?
morphology?

what 3 groups can these be divided into? examples?

Answer 11

multicellular parasitic worms.
have different morphology and sizes at different stages of their lives.
can be microscopic or visible to the naked eye.

Helminths can be divided into 3 groups;
Cestodes are tapeworms e.g. Taenia saginata, the beef tapeworm;
Trematodes are flatworms or flukes and include Schistosoma haematobium, the cause of bilharzia or schistosomiasis
Nematodes which are roundworms e.g. Ascaris lumbricoides.

Question 12

How are microbes visualised?

light microscope - which ones?
electron microscope - which ones?

Answer 12

Bacteria, fungi, protozoa and certain lifecycle stages of helminths - light microscopy

Human viruses at too small to be seen using light microscopy but can be visualised with an electron microscope after special staining (large viruses - light microscopy)

Question 13

Microscopy of bacteria

what happens when suspended in liquid?
what kind of motion is this?

how can bacteria be seen more easily?

Answer 13

Suspended in liquid (broth) - when live, some bacteria can be seen actively migrating from place to place - motile, others seen spinning at the same spot - Brownian movement.

However, bacteria can be seen more easily if they are stained by dyes.

Question 14

Gram stain - different result + colours

positive stain?
negative stain?

what accounts for staining properties?

Answer 14

Gram positive – blue/purple
Gram negative- red

These bacteria have differing cell wall structures which accounts for the staining properties

Question 15

Bacteria have two main morphologies

what are they?

Answer 15

Cocci: these are spherical

Rods or bacilli: These are cylindrical or “sausage” shaped

Question 16

pair, chain or clusters

names for these?

Answer 16

The other thing to note is the arrangement of the bacteria. 
They may be single, 
in pairs e.g. diplococci, 
in chains e.g. streptococci, 
in clusters e.g. staphylococci.

Question 17

Some differences between prokaryotic and Eukaryotic cells

property?
chromosome number?
DNA?
DNA?
histones?
Introns?
nuclear membrane?
Mitochondria?
organelles membrane bound?
sterols?
ribosomes?
reproduction?

Answer 17

Property Prokaryote Eukaryote
Chromosome number one many
DNA circular linear
Histones no yes
Introns absent present
Nuclear membrane absent present
Mitochondria absent present
Organelles membrane bound No Yes
Sterols absent present
Ribosomes 70s 80s
Reproduction binary fission mitosis/meiosis

Question 18

Basic structure of bacterium

what encloses the cytoplasm?
what do the proteins in the cytoplasm do? (2)
what is the cytoplasmic membrane involved in too?

DNA? structure and where?
inclusion bodies?
ribosomes? for?

cell wall? why?

what is outside the cell wall? what does this consist of? function?

what allows independent movement? (2) describe their structure

what short appendages are found on bacteria too? what do these do?

Answer 18

Like the mammalian cell the bacterial cytoplasm is enclosed by a bilayer phospholipid cell membrane or cytoplasmic membrane. The membrane contains a number of proteins which are involved in the transport of solutes from outside the cell. There are also certain proteins in the cytoplasmic membrane which are involved in electron transport associated with cellular respiration

Cytoplasmic membrane is also involved in active secretion of several products of bacteria e.g. proteins and enzymes required for cell wall synthesis, various exotoxins and other enzymes

The cytoplasm houses the bacterial DNA which occurs as a tangled mass without any nuclear membrane (c.f. eukaryotic cell) and is called nucleoid. There are also a large number of ribosomes, consisting of RNA and protein, which are involved in protein synthesis. Bacterial cytoplasm also contains a number of inclusion bodies which are stored sources of nutrition and energy and endospores in sporing organisms.

Outside the cell membrane, the eukaryotic cells have a number of specialised structures which are important for the survival of the cell. Immediately outside the cell membrane, bacteria have a thick and sturdy cell wall which is made of complex molecules and protects the cell membrane and the cytosolic contents. It also maintains the integrity and structure of the bacterial cells.

Outside the cell wall there is often a capsule and slime layer consisting of polymericpolysaccharides. The capsule is normally more organised whilst slime is a loose polysaccharide mass around the cell. These polysaccharides have functions including adherence to other membranes and prevention of phagocytosis. They are also antigenic.

Many bacteria have special appendages which allow them independent movement. These appendages include flagella and axial filaments found in some organisms. Flagella arise from a point in the cell membrane or cell wall called the basal body and may be a single polar, bipolar or multiple around the whole organism. Axial filaments are seen wrapped around spirochetes.

Short filamentous appendages are often found on the surface of some bacteria. These are called pili or fimbriae. These appendages are involved in transfer of genetic material from one organism to another during bacterial conjugation. They also help in the attachment of these bacteria to different mucosal surfaces.

Question 19

gram positve cell wall

composed of what? (2)
what does one of these layers consist of? (2)
What can act here?

Answer 19

It can be seen that the Gram-positive cell wall is composed of a thick peptidoglycan layer and a small amount of a complex polysaccharide called teichoic acid. Peptidoglycan consists of chains of hexoses - N-acetylglucosamine and N-acetylmuramic acid - which are cross-linked by peptide side chains and act as the target site for the action of penicillin type antibiotics..

Question 20

gram negative cell wall

describe the outermebrane
what composes one of these? (2)
what does it play a role in?
what is between the outer mebrane and cell membrane?
special fetaure here?
thin layer of what here?

Answer 20

The Gram-negative cell wall is much more complex, consisting of an outer membrane made of phospholipids, lipopolysaccharides and protein molecules and special protein channels called porins. The lipopolysaccharide that is composed of lipid A and polysaccharide (O antigen) plays an important role as endotoxin. You will learn more about this in the lecture Mechanisms of Bacterial Pathogenesis II. Between the outer membrane and the cell membrane there is the periplasmic space which stores a number of enzymes including, in some bacteria, antibiotic inactivating enzymes. There is also a thin layer of peptidoglycan present in this space.

Question 21

Bacterial Growth

what does it require?
how does bacterial growth occur?
what is generation time? how long usually?
what may grow more slowly/example?

Answer 21

Bacterial growth requires appropriate nutrients essential for growth of a particular organism. In addition, several physiological factors have to be optimum for growth to occur. These include the hydrogen ion concentration (pH) of the growth medium, temperature, osmotic pressure and atmosphere. The chemical requirements include water, carbon source, nitrogen source and minerals.

When the above conditions are satisfied, bacterial growth occurs by binary fission. The cell division occurs in a geometric way, one cell dividing into two, two to 4 and so on. Time taken for one cell to divide to two is called generation time or doubling time and is approximately 20-30 minutes in most bacteria. Some bacteria grow much more slowly e.g. Mycobacterium tuberculosis has a doubling time of 18 hours.

Question 22

BActeria growth - different temps

what do most grow in?
the three different bacteria that grow in different temps, what are they?

Answer 22

The temperature that most pathogenic and commensal bacteria grow well at is 37 oC. Bacteria that can grow in the temperature range 25 oC to 40 oC are described as mesophiles. Bacteria that grow below 20 oC are psychrophiles and those that grow between 55 oC and 80 oC are thermophiles

Question 23

bacteria growth - atmosphere

aerobes - strict grow with or without o2? faculative with or without o2?

anaerobes - strict with or without o2? microaerophilic with or without o2?

Answer 23

Growth in
Class of Bacteria       free o2	absence of oxygen
Aerobes
Strict				+			-
Facultative anaerobe	+			+

Anaerobes
Strict - +
Microaerophilic - +
(can grow with trace of oxygen)

Question 24

BActeria - The Growth Curve

what is the lag phase? why lag?

what is the log phase? what happens during this phase? what does this lead to and why?

what is the last phase?

Answer 24

On introduction into a culture medium, bacterial growth does not start instantly. It takes a while before the bacteria adjust to the new environment before the multiplication can occur. This period varies from bacterium to bacterium and the medium into which they have been introduced. This period of virtually no growth is known as the lag phase.

Bacterial growth then occurs gradually reaching the full growth potential with exponential or logarithmic rate of multiplication - the so called log phase of growth. During this time, maximum utilization of nutrients takes place. In test tubes only a finite amount of nutrient can be provided. When bacterial growth continues at exponential rate, the nutrients are gradually depleted leading to slowing of growth.

During this phase very little growth is recorded and thus is a reflection of the balance between slow growth and some death - stationary phase.

When the nutrient in the test tube is fully exhausted, bacterial growth completely ceases - this is the death or decline phase.

Question 25

Does bacterial growth in vivo mirror in vitro growth to some extent?

what is the lag phase?
log phase?
last two phases?

how is this different in humans?

Answer 25

Lag phase equates to the incubation period, then the Log phase represents the period of establishment of infection. The late Stationary phase and Decline phase represent recovery of host as it reacts against the microbe.

However, this model is probably far too simplistic as there are many differences when comparing a culture with a real infected host.

The mean bacterial generation time is slower in a real patient, nutritional status of patients may vary, redox potential and pH may be different, concentration of metals e.g. iron may be limited by the host, nutrients may be localised and not accessible to the microbe, and probably most importantly the host has cellular and humoral defences as well as physical barrier and chemical defence mechanisms.