Gram Positive Bacteria and Clinical Case Studies II

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Gram Positive BacteriaandClinical Case Studies
II
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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
  diptheria

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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

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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

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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

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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

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Infections

• Erythema
• Cellulitis
• Boils and carbuncles
• Septicemia
• Toxic shock
• Food Poisoning
• Indwelling catheters
• Cystitis

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Streptococcus

• Gram positive
• Non motile
• Ovoid to spherical in shape
• Occur in pairs, short chains, or long chains

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Biochemistry

• Many are facultative anaerobes
• Ferment even in the presence of oxygen
• Require nutrient rich environments
• Identified by their growth patterns on blood agar

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Streptococci

• Group A streptococcus Streptococcus pyogenes
  strep throat
• Group B streptococcus- genital infections
• Endocarditis viridans group
• Otitis, and meningitis by Streptococcus
  pneumonia

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Blood agar Alpha hemolysis
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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

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Serologic

• Lancefields groupings
• This is based upon a carbohydrate, C, in the cell
  wall of the bacteria
• Bacteria are typed according to the variant

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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

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Transmission

• Present in nasopharyngeal region
• Spread via aerosol droplets
• Like sneezing and coughing

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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

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Infections

• Strep throat
• Rheumatic fever
• Acute glomerulonephritis
• Impetigo
• Pharyngitis

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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)

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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

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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

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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

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Novobiocin Test

• Differentiates between staphylococci based upon
  senesitivity to the antibiotic Novobiocin.

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Molecular Methods - PCR
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PCR Results