guideline for interpretation of microbiology lab results · rod clostridium tetani salmonella typhi...

Guideline for interpretation of

Microbiology Lab Results

Lim Kruy, MD (SHCH)

Erika Vlieghe, MD (ITM)

Sample type

Gram stain result

Isolate name

Antibiogram

Gram stain results

A short course on bacteriology

Grampositive Gramnegative

Rod or bacilli

Cocci

Examples?

A short course on bacteriology

Grampositive Gramnegative

Rod Clostridium tetani Salmonella typhi

Coccus Staphylococcus aureus

Meningococcus

Grampositive Gramnegative

Rod

Coccus

Gram positive coccihttp://www.mfi.ku.dk

Staphylococci

•S. aureus

•Coagulase-negative staphylococci

Streptococci

β-hemolytic streptococci

Viridans streptococci

Enterococci

Gram positive Cocci (GPC)• Pairs, chains, Clusters

– Staphylococcus

• Staphylococcus aureus

• Staphyloccous epidermidis----blood/CNS

• Staphylococcus saprophyticus------- urine

• …

• Pairs, chains– Streptococcus and Enterococcus sp

• Pairs– Enterococcus

• Pairs, lancets (or flame)-shaped– Streptococcus pneumoniae (pneumococ)

Hemolytic streptococci Viridans streptococci

Streptococcuspneumoniae

Lancefield groupsA,B, C, D, G, D

GAS= S. pyogenes

Enterococci

Gram Negative Cocci ( GNC)

• Diplococci (pairs, like beans): • Neisseria meningitidis

• Neisseria gonorrhea

• Moraxella catarralis

Gram Positive Bacilli (GPB)

• Diphtheroids– Small, pleomorphic

• Corynebacterium, Propionibacterium

– Large with spores• Clostridium sp• Bacillus sp

– Others: • Listeria sp ( Blood and CSF)

• Lactobacillus sp (vaginal/blood)

– Branching, beaded, rods:• Nocardia sp• Actinomyces sp

Mostly contaminants!

Gram Negative Bacilli (GNB)

• Enterobacteriaceae– Escherichia coli– Klebsiella sp, Enterobacter,

Citrobacter sp, Proteus spp.

– Salmonella , Shigella sp

• Other GNB– Haemophillis influenzae

– Bacteriodes fragilis group

– Fusiform (long, points)• Fusobacterium sp• Capnocytophaga sp

• Non-fermentative rods

– Burkholderia pseudomallei

– Acinetobacter

– Pseudomonas aeruginosa

– …

Difference cannot be made on Gram stain!Identification through growth biochemical properties and typical antibiogram

Gram Negative Bacilli (GNB)

Antibiogram interpretation

1. Look for the name of the bacteria

• Is this a ‘true’ pathogen (causing the disease) e.g.– Staphylococcus aureus

– E coli and other Gram negative bacilli

• Or is this more likely a contaminant e.g.– Coagulase negative Staphylococci

– Bacillus

– Corynebacterium

Choose antibiotics!

No antibiotics needed!!

• if you think your patient needs antibiotics…

2. Look for the first choice antibiotic

• Each bacterium has ‘first choice’ antibiotics

• Best activity, smallest spectrum,…

• Should be written in treatment guideline

• Staphylococcus aureus � cloxacillin

• Pneumococ � penicillin G

• Salmonella species � ciprofloxacin

• E. coli � ciprofloxacin, ampicillin, Augmentin

3. If the first choice AB shows ‘S’

• Then you prescribe the first choice AB!

– According to your local guidelines & availability

– With the smallest spectrum possible

– Dose according to the disease severity e.g.

• S aureus causing skin infection

» Cloxacillin PO 500 mg q6

• S aureus causing blood stream infection/sepsis

» Cloxacillin IV 1-2 g q4-6

4. If the first choice shows ‘R’

• Use the ‘second choice’ antibiotic

– As listed in guidelines

– Or: with known good activity against this bacterium

– AND with the smallest spectrum possible

Example1: Staphylococcus aureus

• Cloxacillin R (MRSA)

– Serious infection� use vancomycin if possible

– Mild infection� use cotrimoxazole (or lincomycin) if ‘S’

Example 2: E. coli

• Ampicillin R � use Augmentin or ciprofloxacin

• Ciprofloxacin R � use Augmentin (if S) or Ceftriaxone

• Augmentin R � check if also Ceftriaxone R (likely ESBL+)

• � use ciprofloxacin, cotrimoxazole (if S)

• � if all other AB R: use meropenem (if available)

• Ceftriaxone R � serious infections: use meropenem (if available)

• � mild infections: use cotrimoxazole, ciprofloxacin (if S)

• � UTI: use nitrofurantoin (if S)

Use only meropenem if serious and if you have no other possibility

ESBL Extended Spectrum Beta-

Lactamase• Lives in bowel bacteria (E.coli, Klebsiella, …)

•Hydrolizes (destroys) all ‘beta-lactam antibiotics’–Penicillin, ampicillin, Augmentin

–Ceftriaxone, cefuroxime and other cephalosporins–Often co-resistant to ciprofloxacin, cotrimoxazole, gentamicin

ESBL• Extended spectrum beta-lactamase

• Treatment options:

– Severe disease:

• carbapenem( meropenem or imipenem)

• amikacin (use ALWAYS in combination)

– Moderate disease:

• Is treatment necessary, is I&D alone enough,…

• Check if you can use cotrimoxazole or ciprofloxacin

• Only if no other option use carbapenem

– UTI: nitrofurantoin (if ‘S’)

Usual duration of treatment• Bacteremia (blood stream infection): 10-14 days

• Typhoid fever: 7-10 days

• Community acquired pneumonia: 7 days

• UTI: 3-5 days• 7 d if nitrofurantoin used

• Skin and soft tissue infection: 7 days

• Meningitis: 10-14 days• 21 d if Streptococcus suis, S. aureus, Listeria, Pseudomonas)

• Osteomyelitis/septic arthritis: 4-6 weeks

Usual doses (1)• Ampicillin

– For (suspected) endocarditis: 1-2 g q4-6 IV– For mild/local infections: 1 g q8 PO

• Augmentin– For respiratory tract: 1 g q8 PO– For urinary/abdominal: 500 mg q6-8 PO– For melioidosis: 1 g q4-6 IV

• Azithromycin– For typhoid fever 500-1000 mg qd PO

• Cloxacillin– For bacteremia/endocarditis: 1-2 g q4-6 IV– For skin/soft tissue infection: 500 mg q6 PO

Usual doses (2)• Ciprofloxacin: 500 mg q12

• Cotrimoxazole– For MRSA and Gram-negative infections: 5/25 mg/kg q12

– For melioidosis: 8/40 mg/kg q12

• Ceftriaxone– For meningitis 2 g qd

– For other indications 2 g q12

• Ceftazidime– For melioidosis: 1 g q8

• Meropenem: 1 g q8 (ONLY FOR SEVERE INFECTIONS)

• Nitrofurantoin: 100 mg q8 x 7 days (UTI ONLY)

Thank you for your attention!