Cephalothin Susceptibility Testing 

Cephalothin is a first-generation cephalosporin with activity primarily against Gram-positive bacteria, including Staphylococcus aureus (methicillin-susceptible), Streptococcus spp., and some Gram-negative bacteria like Escherichia coli and Klebsiella pneumoniae. It is commonly used for soft tissue infections, surgical prophylaxis, and mild urinary tract infections (UTIs). However, cephalothin is susceptible to hydrolysis by β-lactamases, including extended-spectrum β-lactamases (ESBLs) and AmpC β-lactamases, necessitating routine susceptibility testing.

Testing Methodologies

The susceptibility of cephalothin is determined using Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines, employing the following methodologies:

• Etest (Gradient Strip) Method:
  • Utilizes a concentration gradient strip impregnated with cephalothin.
  • Provides direct MIC determination, particularly useful for borderline-resistant strains.
• Automated Susceptibility Testing Systems:
  • Platforms such as VITEK 2, BD Phoenix, and Microscan determine cephalothin MIC values.
  • May require confirmatory BMD testing for borderline-resistant isolates.

Application and Clinical Relevance

• Detection and Monitoring of Resistance Mechanisms:
  • Resistance mechanisms include ESBL production (CTX-M, TEM, SHV), AmpC β-lactamases, and carbapenemases (KPC, NDM, OXA-48, VIM, IMP).
  • Confirmatory ESBL phenotypic testing (e.g., double-disk synergy test or combination disk tests) is essential.
• Surveillance and Resistance Trends:
  • Continuous susceptibility testing is crucial for monitoring emerging resistance patterns.
  • Helps optimize antimicrobial stewardship programs by preventing empirical overuse.
• Role in Empirical and Directed Therapy:
  • Used in empirical therapy for community-acquired soft tissue infections.
  • A critical option for directed therapy in susceptible strains, though not recommended for ESBL or MDR pathogens.

Cephalothin susceptibility testing is essential for diagnostic microbiology and infection control, particularly in Gram-positive bacterial infections. Broth microdilution and disk diffusion methods remain primary approaches, while automated systems offer rapid assessment. Due to emerging resistance mechanisms, continuous surveillance and refinement of MIC breakpoints are necessary to ensure optimal therapeutic decision-making.