11 Jun Investigate the biological basis of antibiotic-resistance
Question
Investigate the biological basis of antibiotic-resistance in a pathogenic bacterium.
Ashley is a healthy, 33-year-old woman. She was admitted to Norfolk General two days ago due to abdominal
cramps, bloody diarrhea, fever, nausea, and vomiting. Ashley’s medical team requested a complete list of all
liquids and foods she consumed the previous day. While reviewing this list, Ashley’s medical team noted that
she consumed a protein shake containing raw eggs prior to her morning training-run. Ashley recently altered
her diet in order to train more effectively for a local triathlon.
Ashley’s medical team immediately ordered the hospital’s analytical team to evaluate the microbes present in
her stool. Particles from her stool were placed on a selective bacterial growth medium that promotes
Enterobacteriaceae growth. The Enterobacteriaceae is a large family of bacteria that includes pathogens such
as Escherichia coli and some Salmonella species.
Colonies of bacteria were visible on this selective growth medium after twelve hours (Figure 1). The Gram
Stain protocol was used to verify that these cultured bacteria are Gram-negative. The Gram Stain protocol also
revealed that these bacteria are rod-shaped bacilli with peritrichous flagella (Figure 2). Flagella are proteinbased filaments emerging from bacteria that provide propulsion and also collect sensory information. In
addition, a metabolic assay showed that these cultured bacteria produce the compound hydrogen sulfide (H2S).
Based on these results, Ashley was treated immediately with penicillin. Her medical team recommended that
Ashley remain in the hospital overnight. When Ashley awoke the next morning, she continued to experience
nausea. This concerned her medical team because penicillin generally eliminates the Gram-negative bacteria
species most commonly associated with food poisoning.
Her medical team immediately ordered the hospital’s analytical team to test the bacteria isolated from Ashley’s
stool sample for beta-lactamase activity. The beta-lactamases are a group of enzymes that degrade the fouratom, beta-lactam ring present in some types of antibiotic molecules. This includes the penicillins, neomycins,
cephamycins, and carbapenems. The degradation of the beta-lactam ring by the beta-lactamases eliminates the
anti-bacterial properties of these antibiotics.
The results showed that the isolated bacteria express the beta-lactamase, TEM-1. This result caused Ashley’s
medical team to adjust her antibiotic therapy. Ashley was treated with the antibiotic, fluoroquinolone, which
eliminated her symptoms. Ashley was released from the hospital the following day.
The TEM Gene codes for the TEM-1 beta-lactamase. The TEM Gene typically resides on a resistance plasmid
(R-Plasmid) rather than on the bacterial chromosome (Figure 3). R-Plasmids carry one or more genes that code
for antibiotic-resistance. A plasmid is a circular DNA molecule that is distinct from the bacterial chromosome.
Plasmids are substantially smaller than the bacterial chromosome. Multiple copies of a single type of plasmid
usually exist in a single bacterium. 1 Figure 1. This image shows colonies of Escherichia coli growing on sheep blood agar
(http://www.retroscope.eu/wordpress/escherichia-coli/). Figure 2. This image shows the variety of flagella produced by rod-shaped, Gram-negative bacteria. 2 Figure 3. This image shows the chromosome and plasmid within a rod-shaped, Gram-negative bacterium.
Plasmid Chromosome Figure 4. This image shows the variation in colony characteristics that exists among bacteria species. 3 1) What is known about Ashley’s illness? 2) What is known about the bacterium isolated from Ashley’s stool sample? 3) What is known about the response of the isolated bacterium to penicillin? 4) What is known about the molecular basis of resistance to penicillin? 5) Based on the evidence, what is your diagnosis of Ashley’s illness? Explain. 6) Which pathogenic bacterium most likely is causing Ashley’s illness? Explain. 7) How would the R-Plasmid that carries the TEM Gene that codes for the beta-lactamase, TEM-1, and
therefore penicillin resistance, complicate Ashley’s treatment? 8) How would the R-Plasmid carrying the TEM Gene that codes for the beta-lactamase, TEM-1, provide
bacteria with an advantage in their natural habitats or in a hospital setting? 4 5
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