Microbiology · Year 2 · from Microbiology
Case 1: Laboratory Identification Challenge
Presentation
A 45-year-old woman presents to the emergency department with fever, chills, and flank pain for 2 days. She reports dysuria and urinary frequency that began 5 days ago. Her past medical history includes type 2 diabetes mellitus and recurrent urinary tract infections. Vital signs show temperature 39.2°C, heart rate 108 bpm, blood pressure 95/60 mmHg, and respiratory rate 22/min.
Blood and urine cultures are obtained. The Gram stain of the urine shows numerous gram-negative rods and white blood cells.
Clinical Image
Gram stain demonstrating gram-negative rod-shaped bacteria (bacilli) - the purple color indicates gram-positive while pink indicates gram-negative organisms. This image shows the characteristic rod morphology of common urinary pathogens.
Image Source: Lecture image - bacterial morphology demonstration
Questions
- Based on the Gram stain findings, what is the most likely category of organisms causing this patient's infection?
- What specific cellular structure explains why this organism appears pink (gram-negative) rather than purple (gram-positive) on Gram stain?
- The laboratory reports the organism is a lactose-fermenting, oxidase-negative, facultative anaerobe. What is the most likely pathogen?
- What mechanism of horizontal gene transfer is most commonly responsible for spread of antibiotic resistance genes in this type of organism?
Answers
- Most likely organism category: The gram-negative rods seen on urine Gram stain, combined with the clinical picture of pyelonephritis and urosepsis, most likely represent Enterobacteriaceae (enteric gram-negative bacteria). The most common urinary pathogens in this family include Escherichia coli (responsible for 80% of community-acquired UTIs), Klebsiella species, and Proteus species.
- Cellular structure explaining Gram stain appearance: Gram-negative bacteria have a thin peptidoglycan layer (2-3 nm) sandwiched between an inner cytoplasmic membrane and an outer membrane. During alcohol decolorization in Gram staining, the crystal violet-iodine complex is washed out through the thin peptidoglycan layer (unlike gram-positive bacteria with thick peptidoglycan that traps the dye). The outer membrane of gram-negative bacteria also contains lipopolysaccharide (LPS), which includes Lipid A - the endotoxin responsible for sepsis pathophysiology.
- Most likely pathogen: Based on the biochemical characteristics described (lactose-fermenting, oxidase-negative, facultative anaerobe), the most likely organism is Escherichia coli. On MacConkey agar, this organism would produce pink colonies due to lactose fermentation. The combination of the clinical syndrome (ascending UTI progressing to pyelonephritis) and these characteristics strongly supports uropathogenic E. coli (UPEC).
- Mechanism of antibiotic resistance spread: Conjugation is the most common mechanism for spread of antibiotic resistance genes among Enterobacteriaceae. Resistance plasmids (R plasmids) carrying genes encoding extended-spectrum beta-lactamases (ESBLs), carbapenemases, and aminoglycoside-modifying enzymes are transferred via the sex pilus during conjugation. This can occur between bacteria of different species, explaining the rapid spread of multidrug resistance in healthcare settings. Transformation and transduction can also transfer resistance genes, but conjugation is most efficient for large plasmid transfer.