Gram Positive BacteriaandClinical 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 diptheria

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

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

Streptococcus

• 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 dropletsLike 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

• 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 ( or )

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