Carbapenem Resistance

Here we will discuss the mechanisms of carbapenem resistance:

  • Enterobacteriacae:
    • Cephalosporinase & porin loss
    • Carbapenemase
  • Pseudomonas aeruginosa:
    • Porin loss
    • Up-regulated efflux
    • Carbapenemase
  • Acinetobacter spp.
    • Cephalosporinase & porin loss
    • Carbapenemase

Here is a diagram of Pseudomonas aeruginosa:



Antibiotic Synergy

In the case of resistant isolates, other methods of disk testing for antibiotic synergy can be employed.

Here, we assess the synergy between linezolid and gentamicin:

And here, synergy between linezolid and rifampicin:



The "L" shape here is important. The shape of the L can determine the level of synergy. If the L shape is more like a triangle then synergy is determined, but if the acute angle is narrow then the antibiotic combination is not synergistic.



Reference Lab ID of meningococci

Meningococcal Isolate Characterisation

  • Phenotypic confirmation of Neisseria meningitidis isolates based on morphology and biochemical reactions.
  • Epidemiological characterisation of strains:
    • Serogrouping: identification of capsular polysaccharide antigens by coagglutination using polyclonal antibodies.
    • Serotyping: identification of (PorB) class 2/3 outer membrane protein by a dot-blot ELISA using monoclonal antibodies.
    • Serosubtyping: identification of (PorA) class 1 outer membrane protein by a dot-blot ELISA using monoclonal antibodies.
    • Molecular characterisation, including non-culture, if required.
  • Antibiotic susceptibility testing:
    • Estimation of penicillin, sulphadiazine, rifampicin and ciprofloxacin MICs are determined for all submitted isolates by agar incorporation method.
    • Other antibiotic susceptibility tests are also available.
  • Molecular subtyping:
    • Molecular subtyping techniques including porA and Multilocus Sequence Typing.
    • (MLST) are being evaluated for routine surveillance. The molecular subtyping techniques are available for epidemiological investigations.


Azithromycin and typhoidal salmonellas

There has recently been some interest in using oral agents after ceftrioxone therapy for the treatment of typhoidal salmonellas. It is widely accepted that in the Indian subcontinent, the levels of ciprofloxacin resistance is increased1. This should be coupled to the fact that in vitro ciprofloxacin susceptibility testing should be backed up with nalidixic acid testing.

The latest BSAC guidelines2, state that azithromycin has been used in the treatment of infections with S. typhi (MIC ≤16 mg/L for wild type isolates) and some enteric infections. Locally we have had a S. typhimurium and an S. paratyphi A, both of which on azithromycin E-testing were found to be 2.0. These have been classed as sensitive.

Duration of treatment: 1 g of azithromycin orally once on the first day, followed by 500 mg given orally once daily on the next 6 days3.


1. Medline


3. Medline