Question

Why should we add Pack-Micro Aero to MRS plates before incubating them and what the differences in this system between Lacto and Campylobcter? and why Campylobacter is incubated at 42 c for 42 h?

Answer 1

MRS agar is a clear brown colored selective medium for the growth of lactobacilli. Sodium acetate in this media will prevent growth of other competitive bacteria. Carbon , nitrogen, and vitamin sources in this media are provided by yeast, meat extracts, and peptone. Polysorbate 80 helps in nutrient uptake by Lactobacilli.

There are two types of packs available: anaeropack-microaero and aneropack-anaero. Pack Micro-aero is added to MRS plate to absorb oxygen from the media in order to maintain oxygen concentration between 6-12% within 1 hour. This system, therefore does not require water or a catalyst. Microorganism such as Campylobacter and Helicobacter are microaerophiles that require low oxygen for growth. Pack-Micro aero will provide this low oxygen concentration for their growth. Pack-Micro aero will also reduce carbon dioxide concentration to 5-8% to allow growth of these microaerophiles. Thus, addition of Pack-Micro aero or anaeropack-anaero (complete loss of oxygen) to MRS plate will allow the growth of lactobacilli in this plate.

Lactobacilli are Gram-positive rods, anaerobic, non-sporulating and catalase negative bacteria. However, these are also aero-tolerant and can grow in presence of oxygen. These organisms can grow on MRS plate in an anaerobic chamber at 37⁰C. However, they can be cultivated with both pack-micro aero and pack-anero as they are usually grown under anaerobic conditions. Lack of oxygen does not affect their growth. Campylobacter will not grow on MRS plate. It is usually grown on Campy CVA plate that contains blood. Further, Campylobacter cannot be cultivated with pack anero system as it is a microaerophile. It requires oxygen for its growth and hence, will not grow under complete anaerobic conditions. Campylobacter grows optimally at 5% Oxygen and 10% Carbon Dioxide.

Campylobacter spp are grown optimally at 42⁰C at48 hr. Its temperature range is between 37⁰C and 45⁰C. Its growth at 37⁰C will be slow, the temperature in humans. Campylobacter spp are usually found in poultry that have a core temperature of 42⁰C. They cause food borne infections in humans. Most human pathogens cannot tolerate this temperature and will be eliminated