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!