Aeromonadales – Isolate Identification, Susceptibility, and Typing

Testing Indications

Medical microbiology laboratories unable to confirm identification of Aeromonadales isolates locally may submit their isolate to PHO for further identification. Laboratories may also submit isolates for susceptibility testing if clinically needed.

Acceptance/Rejection Criteria

Primary specimens, mixed cultures, non-viable cultures, cultures in inappropriate medium, mislabelled cultures, unlabelled cultures, and requisitions with missing information (e.g. primary source of isolation) will be rejected if received.

Storage and Transport

Place the sealed culture in a biohazard bag and properly seal the bag. Cultured isolates must be stored at 2-8°C if they cannot be shipped to PHO on the same day that the incubation is completed.  Refrigerated (2-8°C) cultured isolates must be shipped to PHO on ice packs within 2 days. All cultured isolates must be shipped in accordance to the Transportation of Dangerous Good Act.

Special Instructions

If susceptibility testing is requested, clearly indicate which antibiotic(s) are of interest.

To request additional antimicrobial agents for susceptibility testing after an isolate has been submitted and susceptibility testing already reported by PHO:

1. Determine if and when the susceptibility testing report was finalized at PHO (“Final” written on the report). Additional susceptibility requests on previously tested isolates are only accepted if the request is received within 5 days of the final report.
2. Fill out a new requisition form and indicate in the “Submitter lab no.” field the lab number provided by PHO instead. Under Test(s) Requested section, select “antimicrobial susceptibility” and in “other – (specify)”, write “Additional AST for [name of drug(s)]”.
3. Call PHO Laboratory Customer Service at 416-235-6556/1-877-604-4567 and ask for the appropriate number to fax the completed requisition with the indicated additional susceptibility agents requested.

Test Frequency and Turnaround Time (TAT)

Aeromonadales cultured isolates are processed for identification and/or susceptibility testing daily on weekdays (Monday to Friday) at PHO’s laboratory, Toronto location. If requested, turnaround time for genus or species identification results is up to 4 days from receipt date, and turnaround time for susceptibility results is up to 8 days from receipt date.

Acceptance/Rejection Criteria

Priority testing for cluster/outbreak investigations is available upon request. If needed, contact PHO Laboratory Customer Service as 416-235-6556/1-877-604-4567 prior to sample submission.

Aeromonadales identification of cultured isolates are based on colony morphology, Gram stain morphology, matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS, Bruker RUO systems), and where applicable, a wide range of biochemical and antisera agglutination reactions. Results may be further investigated by 16S targeted Sanger sequencing where applicable.

Susceptibility testing for Aeromonas is performed by minimal inhibitory concentration (MIC) measurement following the Clinical and Laboratory Standards Institute (CLSI) M45. ¹

Algorithm

If identification or confirmation is requested: Cultured isolates are identified at PHO using the identification methods described above.

If susceptibility testing is requested: When the identification provided on the requisition is sufficient to conduct testing, cultured isolates are tested for susceptibility without confirmation of identification. If the identification provided is insufficient, the submitted isolate is first identified at PHO using the identification methods described above. Unless specific agents are requested, antimicrobial agents routinely tested include ciprofloxacin, trimethoprim-sulfamethoxazole, and ceftriaxone by broth microdilution. Additional testing may be offered for select agents if requested.

Interpretation

If identification, confirmation, or typing is requested: Identification will be confirmed to the species level if feasible, otherwise the taxonomic classification most closely aligned to the organism will be reported.

If susceptibility testing is performed: Results will be provided for each antimicrobial agent tested as either “susceptible”, “susceptible dose-dependent”, “intermediate”, or “resistant” according to the applicable CLSI clinical breakpoint for systemic therapy use. MIC values may be provided upon request or if there are no applicable CLSI clinical breakpoints. If the identification provided on the requisition was sufficient, the following will be stated: “Antimicrobial susceptibility testing results and interpretations are based on the submitter’s identification. Identification was not requested and was not performed at PHO.”

As Aeromonas may theoretically harbour an inducible chromosomal ESBL resistance mechanism, the following statement will be added: “Resistance to extended-spectrum penicillins, beta-lactam/beta-lactamase inhibitor combinations, cephalosporins, and aztreonam may develop during therapy with these agents.”²

Reporting

Results are reported to the physician, authorized health care provider (General O. Reg 45/22, s.18) or submitter as indicated on the requisition.

References

1. Clinical & Laboratory Standards Institute. M45 Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria. Wayne, PA.
2. Jacoby GA. AmpC β-lactamases. Clin Microbiol Rev. 2009;22(1):161–82.