A 58-year-old hospitalized patient presents with fever and respiratory deterioration despite broad-spectrum antibiotics. A sputum sample is cultured. Gram stain shows Gram-negative rods. Blood agar yields smooth gray colonies. MacConkey yields yellow colonies. Oxidase yellow; Catalase bubbling; Motility motile; DNase halo; Mannitol agar yellow. Which organism do you suspect and what antibiotic would you switch to?

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Multiple Choice

A 58-year-old hospitalized patient presents with fever and respiratory deterioration despite broad-spectrum antibiotics. A sputum sample is cultured. Gram stain shows Gram-negative rods. Blood agar yields smooth gray colonies. MacConkey yields yellow colonies. Oxidase yellow; Catalase bubbling; Motility motile; DNase halo; Mannitol agar yellow. Which organism do you suspect and what antibiotic would you switch to?

When a hospitalized patient worsens on broad-spectrum antibiotics, think about bacteria with intrinsic resistance patterns and lab clues that point to them. The organism here is a non-lactose-fermenting Gram-negative rod that forms smooth gray colonies on blood agar and yields yellow colonies on MacConkey, indicating it does not ferment lactose. The oxidase test result helps separate common suspects: this organism is oxidase negative, which helps distinguish it from Pseudomonas, which is oxidase positive. The combination of motility, a positive DNase reaction, and ability to ferment mannitol aligns well with Stenotrophomonas maltophilia. This organism is notorious for intrinsic resistance to many antibiotics, including most beta-lactams and carbapenems, which explains the patient’s lack of response to broad-spectrum therapy and makes targeted treatment essential. The appropriate switch is to trimethoprim-sulfamethoxazole, which is the drug of choice for S. maltophilia infections. If TMP-SMX cannot be used, alternatives such as levofloxacin or minocycline may be considered, but TMP-SMX remains the preferred option.

When a hospitalized patient worsens on broad-spectrum antibiotics, think about bacteria with intrinsic resistance patterns and lab clues that point to them. The organism here is a non-lactose-fermenting Gram-negative rod that forms smooth gray colonies on blood agar and yields yellow colonies on MacConkey, indicating it does not ferment lactose. The oxidase test result helps separate common suspects: this organism is oxidase negative, which helps distinguish it from Pseudomonas, which is oxidase positive. The combination of motility, a positive DNase reaction, and ability to ferment mannitol aligns well with Stenotrophomonas maltophilia. This organism is notorious for intrinsic resistance to many antibiotics, including most beta-lactams and carbapenems, which explains the patient’s lack of response to broad-spectrum therapy and makes targeted treatment essential. The appropriate switch is to trimethoprim-sulfamethoxazole, which is the drug of choice for S. maltophilia infections. If TMP-SMX cannot be used, alternatives such as levofloxacin or minocycline may be considered, but TMP-SMX remains the preferred option.

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