Staphylococcus aureus
An introduction to the genetic
features of MRSA
What is a pathogen?
• A pathogen is a disease causing agent
Bacteria
Fungus
Protozoa
Virus
Images: Janice Carr; CDC/Dr. Godon Roberstad; Wellcome Images; Anna Tanczos, Wellcome Images
Question
Can you name a disease caused by
bacteria?
Structure of bacteria
Staphylococcus aureus
• Widespread bacteria:
– Natural flora of the skin of 30% of
the population
• Versatile pathogen associated
with a wide range of diseases:
–
–
–
–
Image: Sharon Peacock, Oxford University
Minor wound infections
Food poisoning
Toxic shock syndrome
Heart infections
How does S. aureus cause disease?
• Bacteria have a range of molecular “weapons”
to help them invade a host and evade
detection.
• What do you think these “weapons” are?
The S. aureus arsenal
Fibronectin binding protein
Collagen binding protein
Toxins
Hemolysins
Leukocidins
Enterotoxins
Exfoliative toxins
Toxic shock syndrome toxin
Clumping factor
S. aureus
Protein A
Elastin binding protein
Matrix adhesion factor
Surface proteins
Enzymes
Lipases
Esterases
Nucleases
Coagulases
Phospholipase C
Extracellular proteins
What is a surface protein?
• Surface proteins are found on the outside of S.
aureus.
• They enable it to attach to host cells aiding
tissue invasion and colonisation.
• Examples:
– Elastin binding protein
– Collagen binding protein
What is a toxin?
• A protein that has the ability to damage
specific cells and cellular components.
Q. Can anyone think of a disease or syndrome
caused by bacterial toxins?
Staphylococcal Scalded Skin Syndrome
• SSSS is an exfoliative dermatitis,
a condition where skin flakes or
peels off.
• Causal Factor : Exfoliative toxins
A and B.
• Can vary in severity ranging
from a few localised blisters to
generalised exfoliation covering
almost the entire body.
Images: CDC
Toxic Shock Syndrome
• Symptoms: sudden onset of
fever, chills, vomiting,
diarrhoea, muscle aches and
rash. Can lead to hypotension
and anaphylactic shock which
can lead to organ failure.
• Causal factor: S. aureus toxic
shock syndrome toxin.
• Associated with tampon use
and also as a complication of
skin abscesses or surgery.
Strawberry tongue symptom of TSS
Image: CDC
What is an enzyme?
• Enzymes break down organic compounds of
cells and tissues, allowing the bacteria to
absorb nutrients and also to spread through
body tissues.
• Examples:
– Protease
– Coagulase
– Lipase
How do you treat S. aureus infections?
• Diseases caused by S. aureus are usually
treated with bactericidal antibiotics and the
patient is normally free of pathogenic bacteria
within a month.
• Antibiotics include:
– penicillin
– oxacillin
– nafcillin
Image: Bmramon/wikimedia commons
Superbugs & antibiotic resistance
• Methicillin resistant
Staphylococcus aureus (MRSA)
is termed a “superbug”.
• Common cause of hospital
acquired infection.
• Hospital-acquired MRSA
infections lead to:
– prolonged hospital stay
– increased costs to NHS
How does resistance occur?
• Use of antibiotics creates a
selective pressure.
• Only bacteria with genes
that confer resistance can
survive a treatment of
antibiotics.
• Eventually resistant
bacteria can make up the
majority of the population.
Population after
Before
antibiotics
Add
antibiotics
antibiotics
Genetic diversity of bacteria
• Bacteria are under continuous selection
pressure and are constantly evolving to adapt
to changes in their environment.
• How?
– Mutations occur within the genome during DNA
copying and because of damage to DNA which
confer a selective advantage.
– Horizontal gene transfer (direct exchange of genes
between individual bacteria).
Horizontal transfer
• New antibiotic genes can be acquired via
horizontal transfer or conjugation.
Images: Genome Research Limited
Viewing diversity
S. aureus comparative genomics
8325
USA300
COL
Mu50
N315
MW2
MSSA476
MRSA252
RF122
0 Mb
1.0 Mb
Staphylococcal cassette chromosome (SCC) element
Genomic islands shared with other S. aureus strains
Pathogenicity islands
2.0 Mb
Integrated plasmid
Prophage
Transposon
Tn916-like element
3.0 Mb
Activity: MRSA gene hunt
• Identify and classify genes from the genomes
of two S. aureus strains.
– MSSA 476: Methicillin susceptible strain but
resistant to penicillin and fusidic acid (commonly
used antibiotics).
– MRSA MW2: Extremely virulent and resistant to
methicillin.
Gene ID cards
Gene ID: gene name
Description of the
protein product and
function
Strain of bacteria
Complete your worksheet
Write protein
product here
Write role or function of
protein here


Answers
Gene
Protein product
Function
MSSA
476
MRSA
MW2
Classification
ccrA & ccrB
Cassette
chromosome
recombinase A & B
Allows new chunks, or “cassettes” of DNA
to be inserted into the bacterial genome.


Mobile genetic
element
ear
Enterotoxin B
Attacks cell membranes which can lead to
the cell bursting and dying as a result.

Toxin
int
(region 3)
Intergrase
Helps insert DNA copied from viral RNA or
DNA into the bacterial genome.

Mobile genetic
element
fusB1
Fusidic acid
resistance protein
Blocks the action of fusidic acid making the
bacteria resistant to common antibiotic skin
creams and ointments.

Antibiotic
resistance
hsdR, hsdS
& hsdM
Restriction
modification
enzymes
Acts as a barrier against ‘free-for-all’
horizontal transfer of DNA protecting the
bacteria’s genome from invading fragments
of DNA which could “weaken” it.

Enzyme
Answers
Gene
Protein product
Function
MSSA
476
MRSA
MW2
Classification
orfX
Surface protein /
gene insertion site
A surface protein which marks the sites on
the genome where mobile genetic
cassettes can insert themselves.


Surface protein
lukS & lukF
Panton-Valentine
leukocidine chain
precursors S & F
Part of the Panton-Valentine leukocidin
complex. When secreted they combine to
form a ring with a hole in the middle,
within the membrane of target cells. This
causes the contents of the cell to leak out
and the cell to die.

Toxin
mecA
Penicillin binding
protein
Blocks penicillin-based antibiotics which
stop synthesis of the bacterial cell wall to
prevent growth and division.

Antibiotic
resistance
Answers
Gene
Protein product Function
sec4
Enterotoxin type c
precursor
sel2
Enterotoxin L
MSSA
476
MRSA
MW2
Classification
A precursor to an enterotoxin. It has to be
modified by another enzyme to become
an active toxin. Once activated the toxin
can damage cells in the gut which leads to
symptoms such as diarrhoea.

Toxin
A type of superantigen capable of causing
a variety of symptoms by destroying the
host’s cells. It stimulates the body to
produce a large number of “T” immune
cells, which damage healthy tissues
around or near the area infected with S.
aureus. This can cause extensive damage
to tissues and major organs sending the
body into shock.

Toxin
Summary
Antibiotic
resistance
Enzyme
Surface protein Toxin
Mobile genetic
element
fusB1
hsdR, hsdS &
hsdM
orfX
ear
ccrA & ccrB
lukS & lukF
int
mecA
sec4
sel2
Key:
MSSA 476 only
MRSA MW2 only
Both strains
Discussion point
• MRSA can be treated with vancomycin.
However the first vancomycin-resistant
strains were discovered in 2003.
• What are the implications of this?