Closing Brief Case: Proteus mirabilis Causing Coraliform Lithiasis and Bacteremia in an Elderly Catheterized Patient

ANSWERS TO SELF-ASSESSMENT QUESTIONS

• 1.
  In which of the following media can swarming phenomenon be prevented?

  • a.
    Culture on blood agar medium.
  • b.
    Culture on CLED medium (cysteine-lactose-electrolyte deficient).
  • c.
    Culture in general medium with low concentration of agar.
  • d.
    Culture in chocolate agar medium.

  Answer: b. CLED medium provides a valuable nonselective diagnostic agar for culture of urinary organisms because it allows the growth of most microorganisms and provides distinct colony morphology. This medium is characterized by being electrolyte deficient to prevent the swarming of Proteus species. L-cystine supports the growth of cysteine-dependent coliform colonies. Lactose is the fermentable carbohydrate, which provides energy and switch from green to yellow if these colonies ferment lactose. The growth in general media such as blood or chocolate would give rise to the swarming phenomenon, avoiding the growth of other microorganisms. This swarming phenomenon is limited by chromogenic media for its high concentration of agar (~4%).
• 2.
  Which of the following statements about the culture and identification of P. mirabilis in chromogenic medium is wrong?

  • a.
    Proteus-Morganella-Providencia are groups of organisms that need confirmatory test of identification by MALDI-TOF.
  • b.
    Chromogenic medium contains artificial substrates (chromogens) that release differently colored compounds upon degradation by specific microbial enzymes.
  • c.
    The culture shows well-defined, creamy-color colonies.
  • d.
    Chromogenic differentiation medium can distinguish P. mirabilis from other Proteus species, especially Proteus penneri.

  Answer: d. Chromogenic differentiation medium (UriSelect 4 Bio-Rad) is a nonselective medium for the identification of urinary tract pathogens. The colonies of P. mirabilis and P. penneri are similar in chromogenic media. The colony grows with a creamy color and an orange halo, inhibiting the swarming phenomenon. The indole test is negative for both microorganisms. To differentiate these Proteus species, it is necessary to use MALDI-TOF. However, Proteus vulgaris shows green color colonies, and the indole test is positive.
• 3.
  To which of the following treatments is P. mirabilis intrinsically resistant?

  • a.
    Ciprofloxacin, ertapenem, ceftriaxone
  • b.
    Ciprofloxacin, fosfomycin, amikacin
  • c.
    Colistin, tigecycline, nitrofurantoin
  • d.
    Nitrofurantoin, levofloxacin, fosfomycin

  Answer: c. P. mirabilis is intrinsically resistant to nitrofurantoin, colistin, and tetracyclines (like tigecycline). P. penneri and P. vulgaris may produce inducible AmpC-type chromosomal β-lactamase. In this case, carbapenem should be used. Third generation of cephalosporins should not be used since they can derepress β-lactamase and generate resistance to cephalosporins.

TAKE-HOME POINTS

• •Proteus mirabilis is one of the main microorganisms belonging to the Morganellaceae family causing urinary tract infection (UTI). This pathogen should be considered especially in patients with CAUTI, institutionalized, with diabetes, with chronic kidney disease, and with recurrent urinary tract infections.
• •In bloodstream infections with septic shock, the focus of urinary origin should always be assessed by imaging diagnostic that exclude a possible obstructive complication, mainly in catheterized patients.
• •P. mirabilis is a facultative fermenting anaerobic Gram-negative rod, urease positive and indole negative. It grows in general media (blood or chocolate agar) with swarming phenomenon. For preventing swarming, specific media such as CLED medium or chromogenic media with a high concentration of agar (around 4%) are used.
• •The treatment of P. mirabilis for urinary tract infection involves the use of a quinolone, cotrimoxazole, amoxicillin/clavulanate, or fosfomycin according to geographic resistance prevalence. In bloodstream infections without extended-spectrum β-lactamase (ESBL), third-generation cephalosporins, aztreonam, or piperacillin/tazobactam should be used. In ESBL strains, a carbapenem could be used.