Microbiology

Spheres in a chain

Streptococci

two spheres

diplococci (eg. streptococcus pneumoniae)

Grape like clusters of spheres

Staphylococci

rod-shaped bacteria

bacilli

spiral shaped bacteria

spirilla eg. Helicobacter pylori
spirochaetes

Explain the difference in cell structure between Gram + and Gram - bacteria and why this causes their respective appearances when stained

Gram positive bacteria have a thick layer of peptidoglycan and only 1 phospholipid membrane

Gram negative bacteria have a thin layer of peptidoglycan, 2 phospholipid membranes and have lipopolysaccharide on their surface

Peptidoglycan soaks up the crystal violet stain. In Gram - bacteria, the alcohol washes away the stain leaving the pink appearance whilst the gram + is too thick so the purple stain remains

Different tests/growth mediums

Gram staining - peptidoglycan/lipopolysaccharide
Ziehl-neelsen - acid fast bacilli (mycobacteria)
Catalase test - streptococci / staphylococci
Coagulase - staphylococcus aureus from other S.
Haemolysis blood agar - various
Chocolate agar - N. meningitidis, gonorrhoea, H.influenzae
Optochin - various
Oxidase - aerobic bacteria
MacConkey agar - gram- bacilli, lactose/non
XLD agar - salmonella / shigella
CLED agar - urinary pathogens
CCDA - campylobacter ?

Staphylococcus aureus:

• shape

• stain

• tests

• diseases/conditions

Grape like clusters

Gram-positive

Coagulase positive

Catalase positive

Beta haemolytic (goldish)

Opportunistic - Abscesses, cellulitis, impetigo, septic arthritis, osteomyelitis

Name the most clinically important streptococci

S. pneumoniae
S. pyogenes
S. agalactiae
Viridans group

Name 3 anaerobic gram positive bacilli

Listeria monocytogenes
Bacillus anthracis
corynebacterium diptheriae

Name 3 anaerobic gram positive bacilli

Clostridium tetani
Clostridium botulinum
Clostridium difficile

Staphylococcus epidermidis:

• shape

• stain

• test

• infection

grape-like clusters
gram-positive
coagulase negative, catalase positive, gamma haemolysis
opportunistic - surgical wounds, in medical devices

Staphylococcus saprophyticus

• shape

• stain

• test

• infection

Grape-like clusters
Gram positive
Coagulase negative, catalase positive, gamma/non haemolytic
Opportunistic - UTIs

Streptococcus pneumoniae

• shape

• stain

• test

• infection

• virulence factors

chain of spheres

Gram +

Catalase negative

alpha haemolytic

optochin sensitive

Opportunistic - otitis media, sinusitis, children/teen/adult/elderly meningitis, most common cause of CA pneumonia

Antiphagocytic capsule, inflammatory wall constituents, cytotoxin

Streptococcus pyogenes

• shape

• stain

• test

• infection

• virulence

Chains of spheres
Gram positive
Catalase negative, Beta haemolytic, lancefield group A
tonsilitis, pharyngitis, otitis, impetigo, cellulitis, scarlet fever
M protein, streptokinase, C5 peptidase reduces chemotaxis, erythrogenic toxins induce inflammation

Streptococcus agalactiae

• shape

• stain

• test

• infections

spheres in chains
Gram positive
catalase negative
Beta haemolytic
Lancefield test group B
Infective endocarditis, meningitis especially newborns, sepsis
opportunistic - affects pregnant women (spread to baby), elderly, immunocompromised and newborns

Viridans group streptococci

• shape

• stain

• test

• infections

chains of spheres
gram positive
catalase negative
alpha haemolytic
optochin resistant
dental abscesses, endocarditis, deep organ abscesses

What are the four major groups of gram-negative pathogens?

Proteobacteria - bacilli + Neisseria, Campylobacter/helicobacter
Bacteroides - bacilli
Chalmydia - round
Spirochaetae - spiral

Escherichia coli

• shape

• stain

• test

• infection

bacilli

gram negative

MacConkey agar pink/red lactose fermenting

CLED yellow lactose

UTI (most common cause of), diarrhoea, cholecystitis, cholangitis, neonatal meningitis (2nd), bacteraemia, septic arthritis, spontaneous bacterial peritonitis

Shigella

• shape

• stain

• test

• infections

Bacilli
gram negative
XLD Agar red/pink colonies NO black dots
negative oxidase test
bloody diarrhoea, shigellosis

Salmonella

• shape

• stain

• tests

• infections

bacilli gram negative

MacConkey agar white non-lactose fermenting

XLD pink/red colonies with black dots

oxidase negative

salmonellosis, diarrhoea, sickle cell osteomyelitis

What bacteria causes cholera?

Vibrio cholerae

Haemophilus influenzae

• shape

• stain

• tests

• infections

• virulence

bacilli

gram negative

chocolate agar growth

pneumonia, meningitis, epiglottitis, sinusitis, otitis media, bacteraemia

anti-phagocytic capsule, resist complement system LPS endotoxin

What bacteria causes whooping cough?

Bordetella pertussis

What bacteria causes gonorrhoea?

Neisseria Gonorrhoeae (2nd most common STD)

What diseases are caused by Neisseria meningitidis?

Meningitis
septicaemia
bacteraemia

Which bacteria is the most common cause of food poisoning in the UK and US?

Campylobacter jejuni

Bacteria which causes gastritis and peptic ulcer disease

Helicobacter pylori

Which bacteria causes lyme disease?

Borrelia burgdoferi (phylum spirochaetae)

What bacteria causes syphilis?

Treponema pallidum

What bacteria causes chlamydia (STD)?

Chalmydia trachomatis

Define a pathogen

Organism that causes or is capable of causing disease

What is meant by a commensal organism?

An organism which colonises the host but causes no disease in normal circumstances

What is an opportunistic pathogen?

An organism which only causes disease when the host defence is compromised

What is virulence?

The degree to which an organism is pathogenic

What is asymptomatic carriage?

When a pathogen is carried harmlessly at a tissue site where it causes no disease

Describe the chain of infection

What does a catalase test demonstrate?

Catalase positive = staphylococci
Catalase negative = streptococci

What separates staphylococcus aureus from other staphylococci?

Staphylococcus aureus is coagulase positive (the rest are negative)

Which bacteria would grow on chocolate agar?

Neisseria meningitidis
Neisseria gonorrhoeae
Haemophilus influenzae

Which bacteria are lance field group A and B?

Group A Streptococcus pyogenes
Group B streptococcus agalactiae

What is the most common bacterial cause of community acquired pneumonia?

Streptococcus pneumoniae

Which bacteria commonly causes dental abscesses?

Viridans group streptococci

What is zoonosis?

An infectious disease that has jumped from non-human animals to humans

What is a virus?

A small infectious agent that replicates inside the cells of other organisms

What is the structure of a virus?

DNA/RNA core, protein coat made up of individual capsomeres, no cytoplasm

Explain the process of viral replication

1. attachment - the virus attaches to a cell by specific receptors

2. entry - the virus enters a cell by endocytosis

3. uncoating - the virus is uncoated to release viral nucleic acid by enzyme degradation

4. replication - once uncoated, replicate by switching the host from cellular protein synthesis to viral

5. assembly - new viral proteins are packaged for release

6. release by lysis (killing cell) or budding (envelope)

How do viruses cause disease?

• direct destruction of host cells

• modify host cells

• over-reactivity of the immune system

• damage through cell proliferation

• evasion of host defences

How does latency aid viruses?

Remaining dormant in cells makes it difficult for the immune system to recognise them

What are the most clinically important mycobacteria?

M. Tuberculosis
M. Avium complex (MAC)
M. Kanasii
M. marinum
M. ulcerans
M. fortuitum
M. Leprae

Why can the onset of tuberculosis take years?

Reproduction rate of tuberculosis is slow 24hr x 2 because maintaining its cell wall is so energy intense
Inactive M. Tuberculosis can be maintained in a stable granuloma for many years until the immune system is compromised

Explain what happens in latent TB

M. Tuberculosis has been metabolically inactive in a stable granuloma with the immune system
Immune system is compromised eg. HIV CD4 count lowers
Granuloma becomes unstable as macrophages are unable to control it
Causes a cavity resulting in symptoms and spread

Why was TB known as consumption?

The low level long term immune response results in strain elsewhere causing low grade fever, weight loss etc.

Which bacteria is responsible for scarlet fever?

Group A streptococcus / streptococcus pyogenes

How does the immune system respond to viral infections?

Antibody response
IgG, IgA and IgM block the binding sites so prevent virus host cell fusion
IgM agglutinates particles
complement system
Interferons have direct antiviral action

How do viruses evade the immune system?

Antigenic drift
Antigenic shift
immune suppression

What is antigenic drift?

Spontaneous mutations causing minor changes in a virus' antigen - results in epidemics

What is antigenic shift?

The sudden emergence of a new subtype of virus different to the preceding because of new antigens - can cause pandemics

How does the body respond to bacterial infection?

Intracellular bacteria - cellular response
Extracellular bacteria - antibody response

How do bacteria defend themselves from the immune system?

Adhesins - bind the bacteria to mucosal surfaces
biofilms - enable bacteria to stick together and protects them from antimicrobials

What is the function of IL-8?

Chemotaxis and degranulation

What is this?

Shingles
Varicella Zoster virus
Maculopapular rash only on one side of the body

What is this?

Chicken pox
Varicella Zoster Virus

What are the 5 main groups of protozoa?

Flagellates
Amoebae
Sporozoa
Ciliates
Microsporidia

What are the different species of malaria?

Plasmodium falciparum
Plasmodium ovale
Plasmodium vivax
Plasmodium malariae
Plasmodium knowlesi

What are the symptoms of malaria?

Fever, chills, headache, sleepiness, loss of appetite, myalgia, fatigue, diarrhoea, vomiting, abdominal pain, anaemia, jaundice, hepatosplenomegaly

What is black water fever?

A complication of malaria in which hemolysis in blood results in Hb release in blood and urine leading to kidney failure

What are the complications of malaria?

Cerebral complications - coma (P. Falciparum)
ARDS
Renal failure
AKI
Bleeding/anaemia
Jaundice
Splenic rupture
Metabolic acidosis

How is malaria diagnosed?

Blood smear under microscope

Describe and explain the lifecycle of the malaria parasite

1. Infected mosquito bites human releasing saliva which contains the parasite

2. parasite is transferred to human as sporozoites

3. sporozoites live and reproduce in hepatocytes

4. They are released into the blood stream as merozoites

5. Merozoites target erythrocytes and form trophozoites (ring structure)

6. Most trophozoites reproduce forming more merozoites which are then released when the RBC is lysed

7. other trophozoites enter a sexual cycle and give rise to gametocytes

8. when an uninfected mosquito bites a human, gametocytes pass into the mosquito

9. the male and female gametocytes fuse to form a zygote, then oocyst

10. the oocyst moves to the salivary glands and releases sporozoites ready to infect again

Difference between primary and secondary vaccine failure

Primary - person doesn't develop immunity
Secondary - initially response but protection wanes over time

What are all the notifiable diseases?

Acute encephalitis, acute infectious hepatitis, acute meningitis, acute poliomyelitis, anthrax, botulism, brucellosis, cholera, COVID-19, diphtheria, enteric fever, food poisoning, haemolytic uraemic syndrome, infectious blood diarrhoea, invasive group A strep disease, legionnaires' disease, leprosy, malaria, measles, meningococcal septicaemia, monkeypox, mumps, plague, rabies, rubella, SARS, smallpox, tetanus, tuberculosis, typhus, viral haemorrhagic fever, whooping cough, yellow fever

Which vaccines are given at 8 weeks?

6 in 1 (diphtheria, tetanus, pertussis, polio, haemophilus influenza type B, hep B)
Meningococcal group B
Rotavirus

Which vaccines are given at 12 weeks?

6 in 1 (2nd dose)
Pneumococcal
Meningococcal group C
rotavirus (2nd dose)

Which vaccines are given at 16 weeks?

6 in 1 (3rd dose)

Meningococcal B (2nd dose)

Pneumococcal (2nd dose)

What vaccinations are given at 1 year?

Hib and Men C (2nd dose/booster)
Pneumococcal (3rd)
MMR
Men B (3rd dose)

When is the influenza vaccine given?

age 2-6 every year

What vaccinations are given at 3 years and 4 months?

MMR (2nd dose)
4 in 1 pre-school booster (diphtheria, tetanus, pertussis, polio)

What vaccines are given at 12/13?

HPV x 2 doses

What vaccines are given at 14?

Tetanus, diphtheria, polio
Meningococcal ACWY

What vaccines are given to people over 50?

50+ flu
60+ shingles
65+ pneumococcal

Difference between endemic, pandemic and epidemic

Endemic - disease found regularly among particular people in a certain area
Pandemic - disease prevalent over a whole country, continent or world
Epidemic - widespread occurrence of an infectious disease in a large population at a particular time

What is meningitis?

Inflammation of the meninges
Specifically arachnoid and Pia mater

How would you tell the difference between a viral and bacterial meningitis infection from CSF?

Bacterial - > high protein, low glucose, PMNs, yellow CSF

Viral - normal protein, normal glucose >50% lymphocytes,

What are the most common bacterial causes of meningitis in newborns?

Group B strep/agalactiae
E. coli
Listeria monocytogenes

What are the most common causes of child/teenage bacterial meningitis?

Neisseria meningitidis
Streptococcus pneumoniae

Which bacteria most commonly cause adult/elderly bacterial meningitis?

Streptococcus pneumoniae
Listeria monocytogenes

What are the common viruses in viral meningitis?

Enteroviruses
herpes simplex virus
varicella zoster virus

Symptoms of meningitis

Headaches
fever
being sick
rash that does not fade when a glass is rolled over it
Neck stiffness
photophobia
phonophobia
unresponsiveness
seizures

Explain the difference between bactericidal antibiotics and bacteriostatic antibiotics

Bactericidal: kill the bacteria
Bacteriostatic: inhibit bacterial growth

What are the different mechanisms of action of antibiotics?

Inhibition of cell wall synthesis
Disruption of cell membranes
Inhibition of DNA/RNA synthesis
Inhibition of protein synthesis
Inhibit folate synthesis (needed for nucleic acids)

Explain how Beta lactams work

Beta lactams bind to Penicillin Binding Proteins (PBPs) causing cell wall lysis

Which drugs inhibit protein synthesis by destroying ribosomes?

Erythromycin (50s)
Chloramphenicol (50s)
Tetracycline (30s)
Streptomycin (30s)
Gentamicin (30s)

Which drugs inhibit folic acid metabolism?

Sulfonamides
Trimethoprim

Which antibiotic groups inhibit cell wall synthesis?

Penicillin
Cephalosporins
Carbapenems

Glycopeptides (not a Beta lactam)

Give examples of common Penicillins

Flucloxacillin
Amoxicillin

Name 2 common glycopeptides

Vancomycin
Teicoplanin

What is the mechanism of metronidazole?

Inhibits protein synthesis, breaks DNA

Name 2 common sulphonamides

Trimethoprim
Co-trimoxazole

What are the common side effects of penicillins?

GI upset
Clostridium Difficile infection
Allergy 10% population