Infection Evaluation Quiz

Gram-Positive Cocci Infections & Fever

• Gram-positive cocci are bacteria characterized by their shape (cocci) and their reaction to Gram staining (positive). This presentation focuses on infections and fever caused by these type of bacteria.
• Slides included images of gram-positive cocci and gram-negative cocci, highlighting the differences in their reactions to gram stains (wear pink on Wednesday).
• The presenter, Robert M. Taylor, PhD, FCCM, presented a case study involving a 52-year-old female with a history of diabetes and a recent knee arthroscopy, experiencing significant knee pain, swelling, and redness for three days.
• Other case study details: fever, chills, and difficulty bearing weight on the affected leg, denies recent trauma or travel history. Blood pressure and other vital signs were also noted.

Disclosures

• No disclosures were made.

Treaty Land Acknowledgment

• The presentation acknowledged the traditional territories of diverse Indigenous groups on which Memorial University's campuses are located.
• Recognition of the Beothuk, Mi'kmaq, Innu, and Inuit peoples and their histories.
• A picture of a Beothuk woman was displayed in the slide.

Case

• A 52-year-old female with diabetes had a left knee arthroscopy.
• Symptoms included severe knee pain, swelling, redness, fever, chills and limited weight bearing.
• No recent travel or trauma were reported.

Laboratory Findings

• White blood cell (WBC) count of 15,000/μL (neutrophilic predominance)
• Elevated erythrocyte sedimentation rate (ESR)
• Elevated C-reactive protein (CRP)
• Pending blood cultures
• Joint aspiration revealed turbid yellow fluid, with a high leukocyte count (85,000/µL) with 90% neutrophils.
• Gram stain indicated the presence of polymorphonuclear cells and gram-positive cocci.

Differential Diagnosis for Septic Arthritis

• The most likely pathogens causing septic arthritis were Staphylococcus aureus, Staphylococcus lugdunensis, group A Streptococcus, group B Streptococcus, Streptococcus anginosus and Enterococcus faecalis/faecium.

Objectives

• Identify the characteristics that distinguish pathogens from contaminants.
• Explain the various stages of bacteriology testing.
• Distinguish characteristics of staphylococci, streptococci, and enterococci species
• Predict organism identity based on select diagnostic findings
• Learn about other clinical characteristics of various bacterial species : S. aureus, S. lugdunensis, S. saprophyticus, Streptococcus pneumoniae, Group A Streptococci, Group B Streptococci and Enterococcus spp., epidemiology, pathogenesis, disease manifestations, diagnostic techniques, first-line treatment and public health responses.

How Humans are Dirty

• Various body sites have different microbial densities and compositions. The mouth has a massive amount of bacteria, followed by the skin (differing levels of density depending on the area and skin folds).
• The lower digestive tract has the highest density of organisms. Sterile sites include the bladder, lungs, liver, etc.

What is Infection?

• Infection is the invasion and growth of germs in the body. Several factors influence infection, including:
• Epidemiology of the pathogen, patient risk factors, pre-analytic factors, analytic factors, and post-analytic factors (tests performed on collected samples of suspected infectious material).

Pre-Analytics

• Proper specimen collection is critical, with volume being an important factor. Multiple sites and sets are recommended for blood cultures.

Analytics (Bacteriology)

• Diagnostic techniques depend on the type of specimen.
• Gram stain is a fundamental tool to guide work-up and treatment.
• Different types of media (enriched, selective, differential) are used according to individual pathogen requirements in order to effectively identify pathogens.

Analytics (Morphology)

• Gram-positive cocci can be single, in pairs (diplococci), chains (streptococci), or clusters of varying shapes.
• Hemolysis (patterns of RBC lysis on blood agar) can help identify specific species of gram-positive bacteria.

Analytics (Biochemical Testing)

• Biochemical testing includes tests like coagulase, catalase, and urease, helps identify bacteria. Commercial techniques like MALDI-TOF (Mass Spectrometry) are increasingly common.

Gram-What?

• The Gram stain is the initial test to determine if an organism is gram-positive or gram-negative, based on cell wall structure. There are fundamental differences in cell wall composition that lead to different outcomes.

Gram Morphologies

• Bacterial species have characteristic shapes, as determined by their cell wall features.

Question 1 (Most Important Pre-analytic Factor for Bacteremia Diagnosis)

• The most important pre-analytic factor is the volume of blood collected for blood culture.

Question 2 (Hemolysis Pattern of S. aureus)

• S. aureus displays a beta hemolysis pattern on blood agar.

Staphylococcus Species

• Coagulase Positive Staphylococci (includes S. aureus)
• Coagulase Negative Staphylococci (includes S. lugdunensis, S. saprophyticus)

S. aureus Complex

• Worldwide distribution
• Common cause of skin infections (SSTIs)
• Commonly found in hospital settings
• MRSA is increasingly difficult to treat.

Pathogenesis

• S. aureus colonizes skin/mucous membranes.
• Invasins, toxins and adhesins contribute to S. aureus pathogenicity.

Clinical Disease

• S. aureus is involved in various localized and systemic infections including Skin infections, bacteremia, pneumonia, endocarditis, SSTIs, and more.
• S. aureus is a key player in causing community-acquired pneumonia.

Diagnostics

• MRSA screening with nasal swabs is a common procedure.
• Gram stains, media (blood agar, mannitol agar), and biochemical tests (coagulase and catalase) are helpful in identifying and differentiating these species.

Question 3 (Key Biochemical Tests for Staphylococci ID)

• Catalase and coagulase.

Antibiotics 101 & Susceptibility Testing

• Antibiotics target different bacterial structures and functions (e.g., cell wall synthesis, protein synthesis, DNA synthesis, metabolic processes).
• Minimum Inhibitory Concentration (MIC) is the lowest drug concentration that inhibits visible growth.

Penicillinases

• Penicillinases are enzymes produced by some bacteria to break down penicillin.
• A nitrocefin disk or penicillin zone edge test can detect the presence of penicillinase activity.

mec A - S. aureus

• mec A encodes a penicillin-binding protein.
• This is associated with methicillin resistance (MRSA).

Susceptibility Testing

• The majority of S. aureus is resistent to penicillin.
• MRSA is resistant to most penicillin-related drugs. Ceftaroline/Ceftobiprole bind differently to the penicillin-binding protein, potentially being used for treating MRSA.

Question 4 (Protein Ceftaroline Binds To in MRSA)

• PBP-2a

S. lugdunensis

• Similar global distribution compared to S. aureus.
• Often associated with infections in sterile sites.
• Important to consider in infections involving native valves.

S. saprophyticus

• Often associated with urinary tract infections (UTIs), particularly in women.

Question 5 (Clinical Feature Associated with S. lugdunensis Infection)

• Native valve endocarditis.

Other Coagulase-Negative Staphylococci (CONS)

• CONS are a heterogeneous group with diverse pathogenic roles.
• The most commonly seen species, S. epidermidis, is mostly a normal flora. However they can also cause infections particularly when seen in the blood stream.

Question 6 (Most Likely Explanation for S. hominis Isolation)

• Contamination during sample collection.

Question 7 (Pathogens of Concern & Treatment in Blood Culture)

• S. aureus and S. lugdunensis are the major potential pathogens. Broad spectrum antibiotic therapy like vancomycin should be used if suspected MRSA, later confirmed via drug sensitivity.

Group B Streptococcus (GBS)

• GBS is commonly found in the lower GI tract and genital tract in many animals.
• It is often asymptomatic in otherwise healthy adults and children.
• During pregnancy, it can cause considerable problems in newborns.
• Chemoprophylaxis with antibiotics can prevent GBS transmission from mother to baby.

S. pneumoniae

• Causes many infections, particularly respiratory infections (pneumonia, otitis media, meningitis).
• Globally ubiquitous and common in the respiratory tract (normal flora).

Question 8 (Most Likely Source of GBS Infection in Newborn)

• Early-onset neonatal sepsis.

Question 9 (Enterococci Species Commonly Associated with VRE)

• E. faecium.

Enterococci

• Enterococci are common gut bacteria that cause a variety of infections, including bacteremia, endocarditis, and UTIs.
• They are frequently resistant to large classes of antibiotics.