USP 51 Antimicrobial Preservative Efficacy

Multidose aqueous products such as injectables, eye drops, cough syrup and others are formulated with preservatives to prevent spoilage from organisms that may be introduced by the consumer.   Amplification of contaminants introduced by the consumer post manufacture could lead to illness at worse and degradation of potency or product functionality at best.

Preservatives are considered an active pharmaceutical ingredient and the “potency” or quantity in a formulation are usually confirmed by HPLC methods before product release and during stability studies.  However, confirming the quantity of preservative by chemical means does not necessarily correlate that the antimicrobial activity of that formulation is still effective.  For this reason, USP <51> is performed during formulation and stability trials.

Aliquots of product are inoculated with test species representing broad microbial groups:

  • P. aeruginosa: Gram negative aerobic rods common in water and soil.
  • E.coli: Gram negative facultative anaerobic rods found in fecal contamination.
  • S. aureus: Gram positive cocci, found on skin and in nares.
  • C. albicans:  Yeast, found in mouth and on skin.
  • A. brasiliensis: Ubiquitous environmental fungus.

Fresh cultures of these test species are introduced to individual aliquots of product at an initial level approximating 1,000,000 cfu/mL.  The contaminated product is tested at weekly intervals, and if the preservative is appropriate, the test species will decrease in counts as indicated below.

.

Category

Products

7 day

14 day

28 day

1

Injections and other parenterals

Bacteria: >1.0 log reduction from initial

Bacteria: >3.0 log reduction from initial

Bacteria: No increase from Day 14

Fungus: No increase from  initial

Fungus:  No increase from initial

Fungus: No increase from initial

2

Topicals, nonsterile nasals products, and those applied to mucous membranes.

 

Bacteria: >2.0 log reduction from initial

Bacteria:  No increase from Day 14

 

Fungus: No increase from initial

Fungus:  No increase from initial

3

Oral aqueous products (non-antacids)

 

>1.0 log reduction from initial

Bacteria :  No increase from Day 14

 

Fungus:  No increase from initial count

Fungus:  No increase from initial count.

4

Antacids

 

Bacteria: No increase from initial count

Bacteria:  No increase from initial count

 

Fungi: No increase from initial count

Fungi: No increase from initial count.

USP <51> Antimicrobial Preservative Effectiveness testing also requires validation – in other words neutralizers which are added to the broth or agar to inactivate the preservatives in the formulation, allowing microbes to grow to visible colony forming units in the agar, must be shown to be efficacious.  Polysorbate and lecithin are typical neutralizers added to media for organic preservatives such as sorbates and parabens.  Halogens may require the addition of sodium thiosulfate.  Regardless, neutralization efficacy is demonstrated by spiking product media mixture with <100 cfu test species, and show 50 – 200% recovery.

Test Requirements:

120 gm or mL of product is ideal for USP <51>.  Total turn around time is 5 – 6 weeks.