Sections View Full Chapter Figures Tables Videos Annotate Full Chapter Figures Tables Videos Supplementary Content +++ MICROBIOLOGY +++ GRAM STAIN EVALUATION ++ Rapid preliminary identification by staining, microscopic evaluation (Table 16-1). Antibiotic therapy may be initiated or broadened based on Gram stain interpretation. Therapy should NOT be narrowed based on a Gram stain. ++ ++Table Graphic Jump LocationTABLE 16-1Gram Stain Evaluation View Table||Download (.pdf) TABLE 16-1 Gram Stain Evaluation Gram-Positive Cocci Gram-Positive Rods Gram-Negative Cocci Gram-Negative Rods Aerobes Streptococcus Enterococcus Staphylococcus Bacillus Listeria Corynebacterium Lactobacillusa Nocardia Neisseria meningitidis Neisseria gonorrhoeae Moraxella catarrhalis E. coli Klebsiella Enterobacter Serratia Citrobacter Morganella Proteus Salmonella Shigella Stenotrophomonas Pseudomonas Pasturella Aeromonas Alcaligenes Burkholderia Pleomorphic coccobacilli: Kingella kingae Acinetobacter Haemophilus influenzae Anaerobes Peptostreptococcus Clostridium spp. Propionibacterium spp. Eubacterium Actinomyces Lactobacillusa Veillonella spp. Bacteroides Prevotella Fusobacterium aAerotolerant anaerobe +++ INTERPRETING AND APPLYING MICs ++ MIC: minimum inhibitory concentration, or the lowest concentration of an antibiotic that inhibits the growth of a particular bacteria (Figure 16-1). Breakpoint: the cutoff that defines susceptibility for an antibiotic and organism pair Breakpoints are determined and updated by the Clinical and Laboratory Standards Institute.1 Breakpoint cutoffs take into consideration MIC distributions, pharmacokinetics, pharmacodynamics, mechanisms of resistance, and clinical outcome data assessing safety and efficacy. MICs should not be compared across different antibiotics in a susceptibility panel. Antibiotic MICs should only be compared to the breakpoint of the same antibiotic. MIC values are not comparable between different antibiotics. For example, if antibiotic A has a lower MIC compared to antibiotic B, that does NOT necessarily indicate antibiotic A is a better choice. The lower a MIC is from the breakpoint of susceptibility, the more likely that antibiotic will be able to achieve the necessary concentration at the site of action. The intermediate susceptibly category serves as a buffer between the susceptible and resistant categories. Infections due to intermediate susceptible organisms may be treatable if higher doses are used or if the antibacterial is concentrated at the site of infection. Limitations and Pearls The choice of antibiotic should be based on the MIC, the breakpoint, and the site of infection. Some MIC breakpoints may differ depending on the site of infection. For example, the breakpoint for ceftriaxone susceptibility when treating Streptococcus pneumoniae in the bloodstream is ≤ 1, whereas the breakpoint for treating the same organism in the meninges is 0.5. Many antibiotics concentrate in the urine and may be used to treat urinary tract infections (UTIs) even when nonsusceptible by in vitro testing. A cephalosporin or penicillin derivative should not be used to treat a serious infection caused by an ESBL (extended-spectrum beta-lactamase)–producing gram-negative bacterium even if considered “sensitive” to cephalosporins per MIC breakpoint. Isolates initially susceptible may become intermediate or resistant after initiation of therapy. This is most frequently seen when a third-generation cephalosporin is used to treat serious Enterobacter, Citrobacter, or Serratia infections, as a result of inducible amp-C beta-lactamases. Interpretation ... Your Access profile is currently affiliated with [InstitutionA] and is in the process of switching affiliations to [InstitutionB]. Please select how you would like to proceed. Keep the current affiliation with [InstitutionA] and continue with the Access profile sign in process Switch affiliation to [InstitutionB] and continue with the Access profile sign in process Get Free Access Through Your Institution Learn how to see if your library subscribes to McGraw Hill Medical products. Subscribe: Institutional or Individual Sign In Error: Incorrect UserName or Password Username Error: Please enter User Name Password Error: Please enter Password Sign in Forgot Password? Forgot Username? Download the Access App: iOS | Android Sign in via OpenAthens Sign in via Shibboleth You already have access! Please proceed to your institution's subscription. Create a free profile for additional features.