Gram-Negative Bacterial

Question 1

Characteristics and locations of Enterobacteriaceae

Answer 1

Characteristics: 
o	Gram negative rod family 
o	Facultative anaerobes
o	Ferment glucose
o	Oxidase negative

Locations:
o Mostly normal flora of GI tract and other sites (vagina)
o Also in environment
o Opportunistic pathogens in immunocompromised hosts

Structure and Serologic classifications:
o All = Somatic O antigens (lipopolysaccharides)
o Some = Flagellar H antigens (confers motility)
o Some = Capsular K antigents (polysaccharides, proteins)

Question 2

Associate the disease syndrome with its associated Gram-negative bacilli

Answer 2

Enterobacteriaceae:
o Sepsis = Escherichia, Klebsiella, Enterobacter, Proteus
o Pneumonia = Klebsiella, Serratia, Enteroacter, Escherichia
o Meningitis = Escherichia
o UTI = Escherichia, Proteus, Providencia
o Gastroenteritis = Salmonella, Shigella, Escherichia, Yersinia
o Intra-abdominal infection = Escherichia

Other Gram-negative Bacteria:
o Bordetella pertussis = tracheobronchitis, whooping cough
o Campylobacter = gastroenteritis
o Haemophilus = upper respiratory infection, otitis media, meningitis, sepsis
o Helicobacter = gastritis, ulcers, gastric cancer
o Moraxella = upper respiratory infection, otitis media
o Neisseria:
• N. gonorrhoeae: genital tract infection, STI
• N. meningitides: nasopharyngeal colonization, meningitis, sepsis
o Pseudomonas = opportunistic lung, skin, eye, burn/wound, blood infections
o Vibrio = gastroenteritis, skin lesions

Question 3

Identify the range of nonpathogenic and pathogenic niches as well as human and nonhuman sources of E. coli.

Answer 3

Nonpathogenic strains:
o Part of normal microbiota in intestine (established in first weeks of life)
o Part of normal facultative microbiota in adult intestine after anaerobes predominate
o Non-human environmental sites:
• Intestines of animals (cows)
• Water and plants in contact with animal waste
o Basic science labs (K-12 EC as molecular biology workhorse)

Pathogenic types:
Intestinal pathogens:
•	Enterotoxigenic EC
•	Enteropathogenic EC
•	Enterohemorrhagic EC
•	Enteroinvasive EC
Extra-intestinal pathogens
•	Uropathogenic EC
•	Neonatal meningitis EC (mainly K1 encapsulated)
Sporadic opportunistic pathogens
•	May cause other disease syndromes (pneumonia, sepsis, intra-abdominal infection)

Question 4

Describe the types of infections caused by E. coli and their clinical significance

Answer 4

Childhood diarrhea
o Major bacterial cause in underdeveloped parts of world
Traveler’s diarrhea
Bacterial enteric pathogen
o 4th leading cause (after Campylobacter, Salmonella, Shigella)
UTI (leading cause)
Neonatal meningitis
o 2nd leading cause (after Group B strep)

Question 5

Identify the 5 types of virulence determinants described in this unit that are commonly shared by bacterial pathogens and provide examples of each.

Answer 5

Adhesion
o Mediated via pili or fimbriae
Types:
Type I Pili
• Attaches to host mannose molecules
• Found in virtually all EC strains
P or PAP (pyelonephritis-associated pilus) Pilli
• Attaches to galactosyl-galactopyranoside (Gal-Gal) molecules
• More often in pyelonephritis vs cystits (bladder) isolates
o Important for kidney colonization
o May have genetic predisposition for UTI (based on expression of particular receptors)
• Cranberry juice = UTI prophylaxis because has receptor blocking molecules

Host Defense evasion
Biofilm production
• Polysaccharide matrix; may incorporate host components like cells and plasma proteins
• Resistance to antibiotics and host defenses
• Help establish and continue infection by pathogens

Polysaccharide capsule
• Anti-phagocytic (large size, negative charge, conceal surface lipids)
• Includes: KI E. coli, Haemophilus influenzae type b, Neisseria meningitides, Strep pneumoniae

Nutrient Acquisition
o Some nutrients (ex: iron) are sequestered by host
o Some bacteria have mechanisms to obtain these nutrients:
Siderophores:
• Low-molecular weight iron-chelating compounds
• Secreted by EC and other bacteria
• Compete with host iron-binding compounds
• Scavenge iron for bacteria
Surface proteins that bind host iron-containing proteins (transferring, lactoferrin)
• Produced by Neisseriae

Mediators of host damage
Toxins
• Secretary toxins
• Subvert natural cellular processes
• Cause hypersecretion of fluid and electrolytes from host
• Ex: cholera toxin produced by Vibrio cholerae
Cytotoxins
• Damage or destroy host cells by different mechanisms
• Examples:
o Shigella = Shiga toxin
o EHEC = Shiga-like toxin or verotoxin
o Helicobacter pylori = vacuolating toxin
o Pseudomonas aeruginosa = exotoxin A
Hydrolytic enzymes
• Degrade host molecules
• Examples:
• Pseudomonas aeruginosa = proteases, lipases, phospholipases
• Haemophilus influenzae and pathogenic Neisseriae = IgA protease (can degrade secretory IgA of host defense)

Other features:
o Acid tolerance = able to survive at low pH
o Ability to grow in urine = trait of many Enterobacteriaceae
o Motility

Question 6

Discuss the 3 genera of Enterobacteriaceae that cause urinary tract infection, including E. coli as the major cause of UTI.

Answer 6

E. coli (uropathogenic EC)
o Leading cause of UTI’s

Proteus spp
o Proteus mirabilis primarily community-acquired
o Other proteus species in hospitals, long-term care facilities, and immunocompromised populations
o Produces urease → hydrolyzes urea to CO2 and ammonia → urinary pH increases = advantageous for Proteus; may foster struvite stone formation and obstruction

Providencia spp
o Usually nosocomial UTI (in catheterized individuals)
o Antibiotic resistance is emerging problem

Klebsiella spp
Serratia spp
Other vaginal colonizers
o	Staph saprophyticus = 2nd leading cause of community acquired UTI in healthy, sexually active women
o	Enterococcus spp
o	Candida albicans (fungus)

Question 7

Describe the role of a polysaccharide capsule as a virulence determinant for bacterial meningitis.

Answer 7

• Capsule is anti-phagocytic (large size, negative charge, conceal surface ligands)
• Specific anti-capsular antibodies can overcome this virulence factor → opsonize → promote phagocytosis

Question 8

Explain how K1 E. coli causes neonatal meningitis.

Answer 8

Predominant etiologic agents of neonatal meningitis: (normal members of vaginal flora)
o #1 bacterial cause: Group B Strep, GBS, S. agalactiae
o #2 cause: E. coli (specifically, K1 EC)

Mechanism of K1 EC:
o K1 capsule is a polymer of sialic acid (abundant on surface of mammalian cells)
o Humans are immunologically tolerant of sialic acid = don’t raise Ab response
o Mother with vaginal K1 EC will not have Abs against K1 capsule → newborn will not have protective maternal Abs against K1 capsule
o K1 capsule can have unfettered antiphagocytic virulence effect

Question 9

Describe the clinical features of pneumonia caused by Enterobacteriaceae such as Klebsiella and Serratia.

Answer 9

Target populations 
o	Community-acquired pneumonia in immunocompromised hosts:
•	Debilitated
•	Elderly
•	Alcoholics
•	IV drug abusers 
•	Underlying lung disease
o	Nosocomial infections  

Agents:
o Klebsiella
• Includes: K. pneumoniae (most common), K. oxytoca, K. oxaenae, K rhinoscleromatis
• Usually a lobar pneumonia (may be severe)
• Progresses to hemorrhagic necrotizing consolidation
• “Currant jelly” sputum (mix of pus and blood)
• Abscess formation is common

o Serratia
• Generally causes nosocmial, opportunistic infections
• May be severe
• Some S. marcescens isolates produce red pigment in culture; but non-pigmented isolates usually cause pneumonia in IV drug users and hospitalized patients

o Others:
• Enterobacter
• Escherichia