Pathogenic Bacteria and Key Diseases

Learning Objectives for Pathogenic Bacteria and Key Diseases

• Conceptual Understanding: Appreciate the concept of the Normal Flora of the human body.
• Mechanisms of Disease: Explain the key mechanisms of bacterial disease pathogenesis.
• Clinical Correlation: Link specific organisms to common clinical syndromes.
• Management Principles: Outline basic diagnostic and treatment principles for bacterial infections.

The Human Normal Flora

Definition and Characteristics

Normal flora consists of organisms that inhabit the body surfaces exposed to the environment. These include the skin, oropharynx, intestinal tract, and vagina.

• Diversity: Members of the normal flora differ in both number and kind depending on the anatomic site.
• Virulence: They are generally low-virulence organisms and remain nonpathogenic in their usual anatomic sites.
• Exclusions: Generally, viruses and parasites are not recognized as part of the normal flora of the human body.

Specific Members of Human Normal Flora by Site

Skin

• Staphylococcus epidermidis: The predominant member. It is a major cause of infections involving prosthetic heart valves and prosthetic joints.
• Staphylococcus aureus: Present on the skin, but its primary reservoir is the nose. It causes abscesses in the skin and various other organs.

Oropharynx (Mouth and Throat)

• Viridans streptococci: The main members of the oral and throat flora. They are the most common cause of subacute endocarditis.

Gastro-intestinal (GI) Tract

• Bacteroides fragilis: An important anaerobic member of the colonic flora that causes disease.
• Escherichia coli: A facultative anaerobe and a critical member of colonic flora causing disease.

Vagina

• Lactobacilli: The predominant organisms in the vagina.
• Function: They maintain a low $pH$ in the vagina, which inhibits the growth of pathogens like Candida albicans (a cause of vaginitis).

Urethra

• Outer Third: Primarily contains a mixture of bacteria including S. epidermidis.
• Female Specifics: The female urethra can become colonized with fecal flora such as E. coli, predisposing individuals to urinary tract infections (UTIs).

Roles and Functions of Normal Flora

Protective Mechanics (Colonization Resistance)

• Nonpathogenic resident bacteria occupy attachment sites on the skin and mucosa.
• They interfere with the colonization of pathogenic bacteria.
• Colonization Resistance: The specific ability of normal flora to limit the growth of pathogens.
• Consequence of Suppression: If flora is suppressed (e.g., by antibiotics), pathogens may overgrow and cause disease.

Nutritional Functions

• Intestinal bacteria produce several B vitamins and Vitamin K.

Pathogenic Potential

• Flora can cause disease in immunocompromised or debilitated individuals.

Circumstances Leading to Disease by Normal Flora

• Barrier Breakdown: Staph aureus causing wound infections after skin trauma.
• Immunosuppression: Conditions such as HIV, chemotherapy, steroid use, malnutrition, and extremes of age.
• Disruption of Flora: Use of broad-spectrum antibiotics leading to overgrowth of resistant organisms like Clostridiodes difficile.
• Translocation: Movement to unusual sites, such as E. coli moving from the gut/perineum to the urinary tract.
• Favorable Growth Conditions: $pH$ changes, hormonal shifts, or nutrient availability (e.g., vaginal candidiasis during pregnancy or after antibiotics).
• Foreign Bodies: Presence of catheters or implants encouraging biofilm formation (e.g., Staph species).

Structure of the Bacterial Pathogen

General Components

A typical bacterial pathogen includes a plasma membrane, cell wall, capsule, pilus (pili), flagellum, fimbriae, cytoplasm, ribosome, nucleoid (DNA), inclusion bodies, and plasmids.

Cell Wall Differences: Gram-Positive vs. Gram-Negative

Gram-Positive Structure

• Peptidoglycan Layer: Thick layer ($15-80\,nm$).
• Teichoic Acid: Present within the peptidoglycan.
• Cytoplasmic Membrane: Internal to the cell wall ($≈ 8\,nm$).

Gram-Negative Structure

• Outer Membrane: Present ($≈ 8\,nm$), containing Lipid A (endotoxin).
• Periplasmic Space: Space between the outer membrane and cytoplasmic membrane, containing hydrolytic enzymes like $̠-lactamases$.
• Peptidoglycan Layer: Very thin ($≈ 2\,nm$).

Other Variants

• Acid-Fast Bacteria: Contain mycolic acid and arabinoglycan above the peptidoglycan layer.

Molecular Composition and Function

Bacterial Disease Pathogenesis

Definitions and Determinants

• Pathogenesis: The process by which a disease or disorder develops.
• Infectious Disease Establishment: Occurs when microorganisms overpower host defenses.
• Determinants of Success:     1. The number of organisms the host is exposed to.     2. The virulence of the organisms.

Stages of Bacterial Pathogenesis

1. Transmission: Entry into the portal of entry.
2. Adherence: Attaching to mucous membranes (essential; non-adherent mutants are non-pathogenic).     * Pili/Fimbriae: Used by N. gonorrhoeae and E. coli.     * Glycocalyx/Slime: Used by S. epidermidis (heart valves/catheters).     * Curli Proteins: Used by E. coli and Salmonella to bind endothelium.     * Capsules: Sticky surfaces that also prevent phagocytosis.
3. Evasion: Avoiding primary host defenses like skin or stomach acid.
4. Colonization: Growth of bacteria at the site of adherence.
5. Disease Symptoms: Caused by toxins, invasion, or immunopathogenesis (inflammation).
6. Host Response: Nonspecific and specific immunity.
7. Resolution or Progression: The outcome of the infection.

Critical Pathogenic Mechanisms

Invasion (The Drill)

• Collagenase/Hyaluronidase: Degrades tissue "cement" (e.g., S. pyogenes).
• Coagulase: Forms fibrin walls to block phagocytes (e.g., S. aureus).
• IgA Protease: Destroys mucosal antibodies (e.g., N. gonorrhoeae).

Toxicity

• Exotoxins (The Sniper): Secreted proteins. Highly toxic and specific (e.g., Botulism neurotoxin, Cholera enterotoxin).
• Endotoxins (The Bomb): Lipid A (LPS) from Gram-negative walls. Released upon cell lysis, causing fever, inflammation, and septic shock.

Immunopathogenesis

Symptoms are caused by the immune response rather than the organism itself.

• Rheumatic Fever: Antibodies against the M protein of S. pyogenes cross-react with joint, heart, and brain tissue.
• Cytokine Storm: Excessive immune response mechanisms.

Stages of Clinical Infection

1. Incubation Period: Silent replication; no symptoms.
2. Prodromal Period: Non-specific symptoms (malaise, fatigue).
3. Specific-Illness Period: Characteristic signs (fever, rash, pain).
4. Outcomes:     * Recovery: Immune clearance and return to baseline.     * Chronic/Carrier State: Persistent, often asymptomatic replication.     * Latency: Dormancy until reactivation (e.g., Herpes, Shingles).

System-Based Pathogens: Cardiac and Bone/Joint

Cardiac Infections: Endocarditis

Endocarditis is an infection of the endocardium, typically affecting the heart valves.

• Diagnosis: Patient history, physical exam, blood cultures (at least two samples taken $12\,hours$ apart), and Echocardiography.
• Treatment: Weeks of intravenous (IV) antibiotics; occasionally surgery.

Other Cardiac Infections

• Myocarditis: Primarily viral (Coxsackie viruses are most common; also CMV, EBV, Parvovirus B19, Influenza). Other causes include Trypanosoma cruzi (Chagas' disease).
• Pericarditis: Inflammation of the pericardium. Mycobacterium tuberculosis is a leading infectious cause worldwide. Also caused by Coxsackie virus, Echovirus, HIV, and CMV.

Bone and Joint Diseases

Osteomyelitis

Infection of the bone, typically at the ends of long bones.

• Clinical Presentation: Bone pain, localized tenderness, fever, night sweats. Acute cases are abrupt; chronic cases are indolent and involve bone necrosis.
• Organisms by Risk Factor:     * Neonates: Streptococcus agalactiae (Group B Strep).     * General Adults/Children: Staphylococcus aureus.     * Prosthetic Joints: Staphylococcus epidermidis.     * Vertebral: S. aureus, M. tuberculosis.     * IV Drug Users: S. aureus, Pseudomonas aeruginosa, Serratia marcescens, Candida albicans.     * Sickle Cell Anemia: Salmonella species.     * Cat Bite: Pasteurella multocida.     * Foot Puncture: P. aeruginosa.
• Diagnosis: Culture of bone aspirate or biopsied tissue (Gold Standard). Blood cultures are positive in $≈ 50\%$ of cases.
• Treatment: Bactericidal antibiotics like Clindamycin (weeks to months). Surgery for chronic cases.

Septic Arthritis

Infection of the joints (synovial fluid).

• Clinical Presentation: Red, warm, swollen, inflamed joint; joint effusion; fever; reluctance to move the joint.
• Pathogens: S. agalactiae (neonates), S. aureus (general), Neisseria gonorrhoeae (sexually active adults).
• Diagnosis: Gram stain and culture of joint fluid (Arthrocentesis).
• Treatment: Prompt antibiotics to prevent joint destruction.

Central Nervous System (CNS) Infections

Bacterial Meningitis

Inflammation of the meninges (membranes covering the brain and spinal cord), typically involving the arachnoid mater and the cerebrospinal fluid (CSF) in the subarachnoid space.

Pathogens by Predisposing Factor

• Neonates: S. agalactiae, E. coli, Listeria monocytogenes.
• Teenagers/Young Adults: N. meningitidis, S. pneumoniae.
• Older Adults: S. pneumoniae, N. meningitidis, L. monocytogenes.
• Spleenectomy/Sickle Cell: S. pneumoniae.
• Complement Deficiency: N. meningitidis.
• Post-Neurosurgery: S. aureus.
• Shunts: S. epidermidis.
• Chronic/Subacute: M. tuberculosis.

Diagnostic Tools

• Imaging: CT or MRI.
• CSF Analysis:     * Gram Stain: Gram-positive diplococci (S. pneumoniae); Gram-negative diplococci (N. meningitidis); Gram-negative coccobacilli (H. influenzae); Gram-positive rods (Listeria).     * Biochemical: Cells (Neutrophils usually indicate bacterial), Protein (elevated), and Glucose (lowered in bacterial).

Urinary Tract Bacterial Infections

Clinical Syndromes

• Cystitis: Bladder infection. Symptoms: Dysuria, frequency, suprapubic tenderness, hematuria.
• Pyelonephritis: Kidney infection. Symptoms: Fever, flank pain, nausea, vomiting, presence of WBC casts in urine.
• Prostatitis: Inflammation of the prostate.     * Younger patients: N. gonorrhoeae, Chlamydia trachomatis.     * Older patients: E. coli.

Pathogens and Diagnosis

• Primary Pathogen: Escherichia coli.
• Other Pathogens: Klebsiella, Proteus, Enterococcus, Staphylococcus saprophyticus (common in young women).
• Urine Dipsticks: Positive for leukocyte esterase and nitrite (nitrite indicates Gram-negative enteric bacteria).
• Urine Culture Threshold: $\geq 1 !!!! imes 10^{5}\,CFU/mL$ from a midstream void is consistent with infection.

Gastrointestinal (GI) Tract Infections

Watery vs. Bloody Diarrhea

Diagnostic Media

• Enterobacteriaceae: MacConkey agar.
• Salmonella enrichment: Selenite F Broth.
• Vibrio cholerae: TCBS.
• Campylobacter: Skirrow medium.

Respiratory Tract Infections

Upper Respiratory Tract

• Otitis Media: Infection of the middle ear. Common bacteria: S. pneumoniae, H. influenzae.
• Pharyngitis: $≈ 10\%$ caused by S. pyogenes (Group A Strep). Risk of rheumatic fever. N. gonorrhoeae is a concern in sexual activity or child abuse.
• Sinusitis: Inflammation of paranasal sinuses. Pathogens: S. pneumoniae, H. influenzae, M. catarrhalis.
• Epiglottitis: Medical emergency. Predominant cause: H. influenzae type B. Treatment: IV Ceftriaxone.

Lower Respiratory Tract

• Bronchitis/Bronchiolitis: Primarily viral (RSV is common in bronchiolitis). Bacteria play a minimal role in acute cases.
• Pneumonia: "Gold standard" diagnosis is an infiltrate on a plain chest radiograph.     * Neonates: Group B Strep, E. coli.     * Young Adults: Mycoplasma pneumoniae, Chlamydophila pneumoniae.     * Specific Exposures: Alcoholism (Klebsiella); Bird exposure (Chlamydophila psittaci); Water aerosols (Legionella).

Skin and Soft Tissue Infections

• Impetigo: Epidermal infection. Pathogens: S. aureus, S. pyogenes.
• Cellulitis vs. Erysipelas: Dermis infections.     * Erysipelas: Superficial dermis, sharply demarcated border.     * Cellulitis: Deeper dermis/subcutaneous fat, diffuse (not sharply demarcated).
• Folliculitis: Superficial hair follicle infection. S. aureus (most common) or P. aeruginosa (hot tubs).
• Abscesses:     * Furuncle: Single infected hair follicle ("boil").     * Carbuncle: Multiple coalesced furuncles, deep subcutaneous involvement.     * Treatment: Primarily incision and drainage.
• Necrotizing Soft Tissue Infections: Medical emergency. Necrotizing fasciitis (fascia) or Myonecrosis (muscle). Can be monomicrobial (S. pyogenes, Vibrio vulnificus) or polymicrobial.

Pelvic and Reproductive Tract Infections

Genital Ulcer Disease

Vaginitis and Other Pelvic Infections

• Bacterial Vaginosis: Malodorous, gray, thin discharge; $pH > 4.5$; "Clue cells" on microscopy.
• Candidiasis: "Cottage cheese" discharge; $pH < 4.5$; pseudohyphae on KOH prep.
• Trichomoniasis: Malodorous green-yellow discharge; $pH > 4.5$; "Strawberry cervix".
• Cervicitis: Inflammation of the cervix. Primarily C. trachomatis or N. gonorrhoeae.
• Pelvic Inflammatory Disease (PID): Polymicrobial infection of the uterus, tubes, and ovaries. Risk of ascending infection from cervix.
• Urethritis: Primarily N. gonorrhoeae (thick purulent discharge) and C. trachomatis (dysuria).