Ceftazidime microbiology
Ceftazidime is bactericidal in action, exerting its effect by inhibition of enzymes responsible for cell-wall synthesis. A wide range of gram-negative organisms is susceptible to ceftazidime in vitro, including strains resistant to gentamicin and other aminoglycosides. In addition, ceftazidime has been shown to be active against gram-positive organisms. It is highly stable to most clinically important beta-lactamases, plasmid or chromosomal, which are produced by bothgram-negative and gram-positive organisms and, consequently, is active against many strains resistant to ampicillin and other cephalosporins.
Ceftazidime has been shown to be active against the following organisms both in vitro and in clinical infections (see INDICATIONS AND USAGE).
Aerobes, Gram-negative
Citrobacter spp., including Citrobacter freundii and Citrobacter diversus; Enterobacter spp., including Enterobacter cloacae and Enterobacter aerogenes; Escherichia coli; Haemophilus influenzae, including ampicillin-resistant strains; Klebsiella spp. (including Klebsiella pneumoniae); Neisseria meningitidis; Proteus mirabilis; Proteus vulgaris; Pseudomonas spp. (including Pseudomonas aeruginosa); and Serratia spp.
Aerobes, Gram-positive
Staphylococcus aureus, including penicillinase- and non-penicillinase-producing strains; Streptococcus agalactiae (group B streptococci); Streptococcus pneumoniae; and Streptococcus pyogenes (group A beta-hemolytic streptococci).
Anaerobes
Bacteroides spp. (NOTE: many strains of Bacteroides fragilis are resistant).
Ceftazidime has been shown to be active in vitro against most strains of the following organisms; however, the clinical significance of these data is unknown: Acinetobacter spp., Clostridium spp. (not including Clostridium difficile), Haemophilus parainfluenzae, Morganella morganii (formerly Proteus morganii), Neisseria gonorrhoeae, Peptococcus spp., Peptostreptococcus spp., Providencia spp. (including Providencia rettgeri, formerly Proteus rettgeri),Salmonella spp., Shigella spp., Staphylococcus epidermidis, and Yersinia enterocolitica.
Ceftazidime and the aminoglycosides have been shown to be synergistic in vitro against Pseudomonas aeruginosa and the  enterobacteriaceae.Ceftazidime and carbenicillin have also been shown to be synergistic in vitro against  Pseudomonas aeruginosa.
Ceftazidime is not active in vitro againstmethicillin-resistant staphylococci, Streptococcus faecalis and many other enterococci, Listeria monocytogenes,Campylobacter   spp., or  Clostridium difficile.

# Susceptibility Tests
Diffusion Techniques
Quantitative methods that require measurement of zone diameters give an estimate of antibiotic susceptibility. One such procedure1-3 has been recommended for use with disks to test susceptibility to ceftazidime.
Reports from the laboratory giving results of the standard single-disk susceptibility test with a 30-mcg ceftazidime disk should be interpreted according to the following criteria:
Susceptible organisms produce zones of 18 mm or greater, indicating that the test organism is likely to respond to therapy.
Organisms that produce zones of 15 to 17 mm are expected to be susceptible if high dosage is used or if the infection is confined to tissues and fluids (e.g., urine) in which high antibiotic levels are attained.
Resistant organisms produce zones of 14 mm or less, indicating that other therapy should be selected.
Organisms should be tested with the ceftazidime disk since ceftazidime has been shown by in vitro tests to be active against certain strains found resistant when other beta-lactam disks are used.
Standardized procedures require the use of laboratory control organisms. The 30-mcg ceftazidime disk should give zone diameters between 25 and 32 mm for Escherichia coli ATCC 25922. For Pseudomonas aeruginosa ATCC 27853, the zone diameters should be between 22 and 29 mm. ForStaphylococcus aureus ATCC 25923, the zone diameters should be between 16 and 20 mm.
Dilution Techniques
In other susceptibility testing procedures, e.g., ICS agar dilution or the equivalent, a bacterial isolate may be considered susceptible if the minimum inhibitory concentration (MIC) value for ceftazidime is not more than 16 mcg/mL. Organisms are considered resistant to ceftazidime if the MIC is 64 mcg/mL. Organisms having an MIC value of 16 mcg/mL are expected to be susceptible if high dosage is used or if the infection is confined to tissues and fluids (e.g., urine) in which high antibiotic levels are attained.
As with standard diffusion methods, dilution procedures require the use of laboratory control organisms. Standard ceftazidime powder should give MIC values in the range of 4 to 16 mcg/mL for Staphylococcus aureus ATCC 25923. For Escherichia coli ATCC 25922, the MIC range should be between 0.125 and 0.5 mcg/mL. For Pseudomonas aeruginosa ATCC 27853, the MIC range should be between 0.5 and 2 mcg/mL.