Gram Positive Organisms
| Methicillin-resistant Staphylococcus aureus (MRSA) | Vancomycin sensitive Enterococcus (VSE) | Vancomycin-resistant Enterococcus (VRE) | E. gallinarum, E. casseliflavus | |
|---|---|---|---|---|
| Definition | Oxacillin minimum inhibitory concentration (MIC) ≥4 mcg/mL | Vancomycin MIC≤4 mcg/mL | Vancomycin MIC≥32 mcg/mL | MIC 8 to 16 mcg/mL |
| Resistance Mechanism | MecA gene encoding for altered penicillin binding protein (PBP-2a) | Van A and Van B genes encoding D-ala-D-lac which replaces D-Ala-D-Ala (vancomycin binding site) in the cell wall | VanC gene encoding intrinsic low-level resistance to vancomycin | |
| Treatment options | Vancomycin# ; daptomycin* ; linezolid; tigecycline; quinupristin-dalfopristin (special access); ceftaroline, ceftobiprole, cotrimoxazole, clindamycin # used for vancomycin susceptible MRSA with MIC ≤2 mcg/Ml * not to be used for MRSA pneumonia due to inhibition by pulmonary surfactant | E.faecalis ampicillin; penicillin G; vancomycin; aminoglycoside* (gentamicin, streptomycin) E. faecium vancomycin; aminoglycoside * monotherapy should not be used; may be combined with a cell wall–active agent for synergy in the treatment of infective endocarditis, if reported as ‘synergism susceptible’ | Linezolid; daptomycin; tigecycline; quinupristin-dalfopristin# ; aminoglycoside* (gentamicin, streptomycin) # only used to treat vancomycin-resistant E. faecium *monotherapy should not be used; may be combined with a cell wall–active agent for synergy in the treatment of infective endocarditis, if reported as ‘synergism susceptible’ | Linezolid; daptomycin; tigecycline; quinupristin-dalfopristin# ; aminoglycoside* (gentamicin, streptomycin) # only used to treat vancomycin-resistant E. faecium; * monotherapy should not be used; may be combined with a cell wall–active agent for synergy in the treatment of infective endocarditis, if reported as ‘synergism susceptible’ |
Gram Negative Organisms
| ESBL | ampC | Carbapenemase | |
|---|---|---|---|
| Definition | Class A β-lactamase which is resistant to all β-lactams except carbapenems, cephamycins (cefoxitin, cefotetan, cefmetazole), cefipime and ß-lactam/ ß -lactamase inhibitor combinations | Class C β-lactamase, the product of the ampC gene, which is resistant to all β-lactams except carbapenems and cefipime. | Carbapenem-hydrolyzing beta-lactamase, which is resistant to a broad spectrum of beta-lactams including carbapenems. |
| Common organisms | Most commonly found in E. coli and Klebsiella spp. but also in other gram negative bacteria. | Plasmid-mediated ampC: E. coli, K. pneumoniae, and Proteus. mirabilis Chromosome-mediated ampC: SPICE organisms (Serratia spp., Proteus vulgaris/penneri, Providencia, all Citrobacter spp. except C. koseri, Morganella morganii, Enterobacter spp., Hafnia alvei, Pantoea agglomerans, Pantoea dispersa) | Klebsiella pneumoniae carbapenemase (KPC): Class A β-lactamase, found in Enterobacteriaceae. Metallo-beta-lactamases (MBLs): Class B β-lactamase, the New Delhi MBLs (NDM-1) was found in Enterobacteriaceae and Acinetobacter. OXA carbapenemases: Class D β-lactamase, found in acinetobacter and Enterobacteriaceae. |
| Treatment options | 1st : carbapenems Others depending on susceptibility testing results: ciprofloxacin, aminoglycoside, septra, fosfomycin (only for UTI) | 1st : carbapenems Others depending on susceptibility testing results: ciprofloxacin, aminoglycoside, septra, fosfomycin (only for UTI) | colistin, polymyxin B, aztreonam, tigecycline, fosfomycin (only for UTI). Frequent resistance to aminoglycosides and fluoroquinolones |
Pseudomonas Aeruginosa
| Organism | Aerobic, motile, straight, slender, Gram negative bacilli |
|---|---|
| Colonial morphology | On blood agar: rough, most often beta-hemolytic with bluish green, red or brown pigmentation; concord grapes or corn tortilla smell with metallic sheen; mucoid colonies commonly seen in patients with cystic fibrosis. On MacConkey agar: colorless colonies |
| Presumptive identification | Non-lactose fermenter, oxidase +, oxidize glucose, able to grow at 42 °C |
| Therapeutic Options | ß-lactam/ ß -lactamase inhibitor combinations: Piperacillin/tazobactam Cabapenems: meropenem, imipenem (not ertapenem) Aztreonam Cephalosporins: Ceftazidime, Cefepime Fluroquinolones: Ciprofloxacin>levofloxacin Aminoglycosides |
Risk Factors For Resistant Organisms
| MRSA | VRSA | VRE | CPE (Carbapenemase-producing organisms) | Acinetobacter |
|---|---|---|---|---|
| Antibiotic use (esp. cephalosporin and fluoroquinolone) | Age >50 years | Antibiotic use (esp. cephalosporin, vancomycin ) | Use of broad spectrum cephalosporins, carbapenems | Use of beta-lactam use, esp. carbapenems, fluoroquinolone |
| HIV infection | Vancomycin for >48 hours in the week prior to bacteremia | Significant underlying medical conditions | Significant underlying medical conditions | Prior colonization with MRSA |
| Hemodialysis | Chronic liver disease | Colonization pressure | Trauma | Mechanical ventilation |
| Residents of long-term care facilities | History of MRSA bacteremia, central venous catheters (CVL) | Exposure to contaminated surfaces | Mechanical ventilation, CVL | Bedridden status, indwelling catheter |