Gram Positive Bacteria
and
Clinical Case Studies II
Introduction
• Staphylococci and streptococci
constitute the main group of
medically important gram positive
bacteria
• There are also bacilli that are
pathogenic such as Anthrax and
the Clostridia group. These are
also spore forming organisms
• There are also some other gram
positive rods such as Listeria,
Lactobacillus,and Cornybacterium
Staphylococci
• Staphylococci stain dark purple with the
gram stain.
• There are three medically important
forms of staphylococci
• Staphylococcus aureus – this causes
many types of infections, food
poisoning,and toxic shock
• Staphylococcus epidermidis – this is a
cause of infections in indwelling
catheters
• Staphylococcus saprophyticus that is an
ongoing cause of cystitis in women
Staphylococci
• Found in the air and on surfaces
• Very resistant to drying and dehydration
• They produce Catalase which is one of their
distinguishing characteristics* this is an
enzyme vital to the survival of many aerobic
bacteria
• The most virulent form of staphylococcus, SA
also secretes coagulase, that causes citrated
plasma to clot. These are referred to as
coagulase positive
• There are other staph that do not have this
ability and are labeled coagulase negative
Staphylococcus aureus
• Carried by 20-40 % of healthy
individuals in their mouths or on
their skin
• It also found in their nasal mucosa
• Carriers can serve as a source of
infections as well as inanimate
objects – these are called fomites
Staphylococci and virulence factors
• Protein A – antiphagocytic effect
• Exotoxins – Can cause cell lysis and
destruction
• Enterotoxins – Occurs when
bacteria release toxin and cause
vomiting
• Toxic shock
• Exfoliative –skin eruption and
destruction
Infections
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Erythema
Cellulitis
Boils and carbuncles
Septicemia
Toxic shock
Food Poisoning
Indwelling catheters
Cystitis
Streptococcus
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Gram positive
Non motile
Ovoid to spherical in shape
Occur in pairs, short chains, or long
chains
Biochemistry
• Many are facultative anaerobes
• Ferment even in the presence of
oxygen
• Require nutrient rich environments
• Identified by their growth
patterns on blood agar
Streptococci
• Group A streptococcus –
Streptococcus pyogenes – strep
throat
• Group B – streptococcus- genital
infections
Endocarditis – viridans group
Otitis, and meningitis by
Streptococcus pneumonia
Blood agar – Alpha hemolysis
Hemolytic bacteria
• Cause a biochemical change in the
hemoglobin of red blood cells –
alpha hemolysis – green around the
colonies
• Cause gross hemolysis of the red
blood cells in blood agar – beta
hemolysis
• Gamma – no hemolysis or change in
the blood agar
Serologic
• Lancefields groupings
This is based upon a carbohydrate, C,
in the cell wall of the bacteria
Bacteria are typed according to the
variant
Streptococcus pyogenes
• Gram positive, non motile
• Requires a low inoculum for
infections
• It does not survive well in the
environment
• Invades mucous membranes
• Rapid progression of infection
• Post infection sequelae can lead to
glomerulonephritis
Transmission
• Present in nasopharyngeal region
• Spread via aerosol droplets
Like sneezing and coughing
Pathogenic features
• Fimbriae – for attachment- M
protein
• Exotoxins – cause rashes and other
skin effects
• Cytolytic toxins
• Streptolysins – lyse white blood
cells and red blood cells
• Hyaluronidase – breaks down
connective tissue to spread
infection
Infections
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Strep throat
Rheumatic fever
Acute glomerulonephritis
Impetigo
Pharyngitis
Summary of differences between
staphylococci and streptococci
• Gram stain and morphology
– Both Gram +
– Staphylococci: bunched cocci
– Streptococci: chained cocci (S. pneumoniae form
diplococcus)
• Enzyme tests
– Staphylococci: catalase +
– Streptococci: catalase -
• Growth
– Staph.: large colonies (non-fastidious), some
hemolytic
– Strep.: small colonies (fastidious), many hemolytic
(a or b)
Serological Techniques
• Lancefield classified the
streptococci into 20 serotypes
designated A through V.
• This is based upon an antigeninc
substance called C
• Streptococcus pyogenes belongs to
Group A
• These are also beta hemolytic
Bacitracin
• A filter paper disc impregnated
with the antibiotic Bacitracin is
placed on a blood agar plate
containing bacteria
• If there is a zone of inhibition
around the disk it is indicative of a
Group A streptococci
Serological technique
• Directigen test – A rapid
nongrowth dependent immunological
procedure
• The bacteria are mixed with
reagents that extract the Group A
antigen and produce positive test
results
Novobiocin Test
• Differentiates between
staphylococci based upon
senesitivity to the antibiotic
Novobiocin.
Molecular Methods - PCR
PCR Results