Gram-Negative Bacteria – Exam 3 Review

A comprehensive set of question-and-answer flashcards covering the structure, taxonomy, and medically important members of Gram-negative bacteria, including Proteobacteria and key Non-proteobacteria phyla. These cards review pathogens, ecological roles, toxins, clinical syndromes, and prevention points relevant for Exam 3.

What structural features define Gram-negative bacteria?

A thin (≈10 %) peptidoglycan layer and an outer membrane that contains lipopolysaccharide (LPS, an endotoxin).

During a Gram stain, why do Gram-negative cells appear pink?

They cannot retain the crystal-violet–iodine complex after alcohol decolorization and instead take up the counter-stain (safranin/fuchsin).

Which outer-membrane molecule of Gram-negative bacteria acts as endotoxin?

Lipopolysaccharide (LPS).

What molecular marker is commonly used to infer bacterial phylogeny?

The 16S rRNA gene sequence.

Into what two broad groups are Gram-negative bacteria divided based on 16S rRNA phylogeny?

Proteobacteria and Non-proteobacteria.

How many recognized classes are in the phylum Proteobacteria?

Five: α, β, γ, δ, and ε Proteobacteria.

Which bacterial phylum contains the largest phylogenetically coherent group of ‘classic’ Gram-negatives?

Proteobacteria.

Name two medically relevant genera within the α-Proteobacteria class.

Rickettsia and Rhizobium/Agrobacterium (Rhizobiaceae).

What disease is caused by Rickettsia rickettsii and how is it transmitted?

Rocky Mountain Spotted Fever; primarily transmitted by ticks.

Which Rickettsia species causes epidemic typhus and what are its vectors?

Rickettsia prowazekii; spread by body lice and fleas.

Which α-Proteobacterium forms nitrogen-fixing root nodules on legumes?

Rhizobium spp.

What plasmid allows Agrobacterium tumefaciens to cause crown-gall disease?

The tumor-inducing (Ti) plasmid.

Name two pathogenic Neisseria species (β-Proteobacteria).

Neisseria gonorrhoeae (gonorrhea) and Neisseria meningitidis (meningococcal meningitis).

Which β-Proteobacterium causes whooping cough?

Bordetella pertussis.

What opportunistic β-Proteobacterium endangers cystic-fibrosis patients?

Burkholderia cepacia complex (often simply Burkholderia).

Which class of Proteobacteria contains the Enterobacteriaceae, Vibrionaceae, and Pseudomonadaceae?

γ-Proteobacteria (Gamma).

Why are Enterobacteriaceae called “enterics”?

Because many inhabit the intestinal tracts of vertebrates.

List four primary enteric pathogens.

Yersinia, Salmonella, Shigella, and pathogenic E. coli (e.g., O157:H7).

Which bacterium caused historic bubonic and pneumonic plague?

Yersinia pestis.

What toxin makes E. coli O157:H7 especially dangerous?

Shiga toxin 2 (Stx2) acquired via lysogenic bacteriophage.

What severe complication can follow E. coli O157:H7 infection?

Hemolytic uremic syndrome (HUS).

Why can raw flour be a vehicle for E. coli O157:H7 outbreaks?

Flour is a raw agricultural product that is not heat-treated and can be contaminated by animal feces or environmental pathogens.

Which genus is the major hygienic indicator for fecal contamination of food and water?

Escherichia coli (coliform group).

Which γ-Proteobacterium is the #1 cause of pediatric bacterial meningitis before widespread vaccination?

Haemophilus influenzae type b (Hib).

What lysogenic phage gene must Vibrio cholerae carry to cause cholera?

The cholera toxin gene (ctx).

Which marine Vibrio species can cause necrotizing fasciitis when wounds contact seawater?

Vibrio vulnificus.

Who is considered the father of modern epidemiology for tracing a cholera outbreak to the Broad Street pump?

Dr. John Snow.

Which pigment-producing γ-Proteobacterium is a major multidrug-resistant nosocomial pathogen, especially in burn units?

Pseudomonas aeruginosa (produces pyocyanin, pyoverdin, pyorubin).

What disease does Legionella pneumophila cause and where is it often acquired?

Legionnaires’ disease; from inhaling aerosols from hot tubs, plumbing systems, fountains, or AC units.

Which δ-Proteobacterium preys on other bacteria in the periplasm and is dubbed a “living antibiotic”?

Bdellovibrio.

What social, swarming δ-Proteobacteria hunt in “wolf packs” and form myxospores?

Myxobacteria.

Name two pathogenic ε-Proteobacteria and the illnesses they cause.

Campylobacter jejuni (enteric infection with bloody diarrhea) and Helicobacter pylori (peptic ulcers, gastritis, stomach cancer).

Approximately what percentage of healthy humans carry Helicobacter pylori?

About 50 %.

Which form of Chlamydia is infectious in the environment?

The elementary body (small, extracellular, infectious form).

What human diseases can Chlamydia trachomatis cause?

Sexually transmitted infections and trachoma (leading to blindness).

Which phylum contains spiral bacteria with internal endoflagella (axial filaments)?

Spirochaetes.

What tick-borne spirochete causes Lyme disease?

Borrelia burgdorferi.

What is the characteristic early skin lesion of Lyme disease?

Erythema migrans (bull’s-eye rash).

Which spirochete causes syphilis?

Treponema pallidum.

Describe the painless lesion of primary syphilis.

A chancre that appears ≈3 weeks after exposure and often goes unnoticed.

What Gram-negative phylum dominates the human gut and tonsillar crypts as strict anaerobes?

Bacteroidetes (notably the genus Bacteroides).

Why can Bacteroides species become pathogenic inside the abdominal cavity?

They are strict anaerobes from the gut; if they enter normally sterile sites via wounds or ruptured appendix they cause infection and carry many antibiotic-resistance genes.

Which extremophile phylum provides Taq DNA polymerase used in PCR?

Phylum Thermus (part of Thermus-Deinococcus group).

What unique ability allows Deinococcus species to survive high gamma radiation?

Efficient DNA repair that reassembles a fragmented genome within 12–24 hours.

What type of photosynthesis do green sulfur bacteria (Chlorobi) perform?

Anoxygenic photosynthesis (do not split water; no O₂ produced).

Which phylum of oxygenic photosynthetic bacteria is also called “blue-green algae”?

Cyanobacteria.

What enzyme do cyanobacteria use for carbon fixation?

RuBisCO.

Why are cyanotoxins a concern in Florida waters?

Warm, nutrient-rich lakes and rivers (e.g., Lake Okeechobee) promote cyanobacterial blooms that release neurotoxins, sometimes coinciding with red tide (Karenia brevis) for compounded health/ecological damage.

Which hyperthermophilic phyla show extensive horizontal gene transfer from Archaea?

Aquificae and Thermotogae.

What is the infectious reservoir for Campylobacter jejuni commonly found in retail food?

Almost all retail chicken meat.

Which antibiotic-resistant enteric pathogen can cause severe dysentery with as few as 10–100 cells?

Shigella spp.

What simple household practice is repeatedly emphasized to prevent fecal-oral transmission of Shigella and other enterics?

Wash your hands thoroughly.

What cleaning method is recommended for fresh produce instead of soap or commercial veggie washes?

Rinse under running water with friction (hands or brush) and dry with a paper towel.