1) The document discusses intrinsic or inherent resistance in microorganisms and guidelines for reporting antimicrobial susceptibility test results. Certain organism-antimicrobial combinations should not be reported as susceptible even if in vitro testing shows susceptibility due to intrinsic or innate resistance. 2) Examples given include that Salmonella and Shigella should not be reported as susceptible to certain drug classes that are intrinsically ineffective clinically. Additionally, certain genera like Enterococcus and Staphylococcus have intrinsic resistance to specific antimicrobial classes. 3) The document provides categories of intrinsic resistance for various bacteria like Enterobacteriaceae, non-fermenting Gram-negatives, and Gram-positive cocci to assist in accurate reporting of susceptibility test results

1. Inherent resistance amongst
microorganism: how to report
AST Guideline
Tuba Bashir

2. Intrinsic Resistance
 Intrinsic resistance is defined as inherent or innate (not acquired)
antimicrobial resistance, which is reflected in wild-type antimicrobial patterns
of all or almost all representatives of a species. Intrinsic resistance is so
common that susceptibility testing is unnecessary. For example, Citrobacter
species are intrinsically resistant to ampicillin.
1) they provide a way to evaluate the accuracy of testing methods;
2) they aid in the recognition of common
phenotypes; and
3) they can assist with verification of cumulative antimicrobial susceptibility
test data.
 In the tables, an “R” occurring with an organismantimicrobial agent
combination means that strains should test resistant.
 A small percentage (1% to 3%) may appear susceptible due to method variation,
mutation, or low levels of resistance expression.

3. Inherent resistance : how to report
1. Enterobacteriaceae
 For Salmonella and Shigella DO NOT report :
 aminoglycosides,
 first or second generation cephalosporins (cefazolin/ cefuroxime)
or cephamycins (cefoxitin)
as isolates are susceptible in vitro but NOT EFFECTIVE CLINICALLY
and SHOULD NOT BE REPORTED
 Proteus/ Providencia/Morganella (Proteus group) :
 have elevated IMIPENEM MICs. MEROPENEM is the preferable
carbapenem. However, if Imipenem tests susceptible, results may
be reported as such
 Enterobacteriaceae are intrinsically resistant to
vancomycin, daptomycin, linezolid, clindamycin,
macrolides, fusidic acid and rifampicin

4. Inherent resistance : how to report
Non-enterbacteriaceae
 Non-fermentative gram negatives are
INTRINSICALLY RESISTANT TO :
 cefazolin, cefoxitin, cefuroxime, penicillin, vancomycin,
daptomycin, linezolid, clindamycin, macrolides, fusidic
acid and rifampicin
 Stenotrophomonas is intrinsically resistant to :
 tetracycline, but not to doxycycline or minocycline

5. Inherent resistance : how to report
Staphylococci and Enterococci
 Grampositive bacteria are intrinsically resistant to
polymyxins, aztreonam, and nalidixic acid
 For oxacillin resistant staphylococci, other beta
lactams must not be reported as susceptible.
Ceftaroline is a notable exception (not currently
available for testing)

6. Inherent resistance : how to report
Enterococci
 For Enterococcus spp, SHOULD NOT BE
REPORTED
 cephalosporins, aminoglycosides (except high-level
resistance screening), clindamycin and cotrimoxazole
may appear active in vitro but are ineffective clinically
and Enterococcus faecalis is inherently
quinupristin/dalfopristin resistant
 Enterococcus gallinarum/casseliflavus are
intrinsically resistant
 vancomycin and quinupristin/dalfopristin