🔴 Cronobacter spp. (milk powder)

Microbiology PN-EN ISO 22964

Overview

Cronobacter spp. (formerly Enterobacter sakazakii) is an opportunistic pathogen posing serious threat to newborns and infants, especially premature babies and children with reduced immunity. Cronobacter infections can lead to meningitis, septicemia and necrotizing enterocolitis — mortality reaches 40–80% in newborns. The main source of infection is contaminated milk powder (powdered infant formula — PIF).

The PN-EN ISO 22964 standard describes a horizontal method for detecting Cronobacter spp. in food, with particular focus on milk powder, infant formula and environmental samples from facilities producing these products. The method includes three stages: pre-enrichment in buffered peptone water (BPW), selective enrichment in modified lauryl sulfate tryptose broth with vancomycin (mLST/vancomycin), and isolation on chromogenic agar (CCI — Chromogenic Cronobacter Isolation Agar).

Cronobacter shows characteristic α-glucosidase activity, which hydrolyzes chromogenic substrate in CCI agar, forming blue-green (or turquoise) colonies. Vancomycin in mLST broth inhibits Gram-positive bacteria growth, enabling selective Cronobacter enrichment.

Cronobacter is particularly dangerous because it survives in low water activity environments (milk powder, flours, herbal teas). Testing is required by Regulation (EC) No 2073/2005 for powdered infant formula (criterion: absence in 10 g). "Detected" result leads to product batch withdrawal from market.

Method principle

Three-stage method: (1) Pre-enrichment — 25 g sample in 225 mL BPW, incubation 37°C/18 h — resuscitation of damaged cells. (2) Selective enrichment — 0.1 mL transfer to 10 mL mLST with vancomycin, incubation 44°C/24 h — Cronobacter selection (thermophilic growth), vancomycin inhibits Gram-positives. (3) Isolation — plating on chromogenic CCI agar (Chromogenic Cronobacter Isolation Agar), incubation 44°C/24 h. Cronobacter produces α-glucosidase, which hydrolyzes chromogenic substrate — blue-green colonies. Confirmation: sugar fermentation, yellow pigmentation on TSA, biochemical or molecular identification.

Applications

• Infant milk powder (PIF) quality control — Regulation (EC) 2073/2005
• Powdered infant formula testing
• Environmental monitoring of milk powder production facilities
• Raw materials for infant formula production control
• Nutritional supplements and food for special medical purposes testing
• Herbal teas, starches and flours monitoring (Cronobacter reservoirs)
• Production processes and cleaning systems (CIP) validation

Key parameters

Standard

Standard number

PN-EN ISO 22964:2017

Title (PL)

Mikrobiologia łańcucha żywnościowego — Horyzontalna metoda wykrywania Cronobacter spp.

Title (EN)

Microbiology of the food chain — Horizontal method for the detection of Cronobacter spp.

Step-by-step procedure

1. Sample preparation

   Weigh 25 g milk powder (PIF) into stomacher bag. Add 225 mL buffered peptone water (BPW) preheated to 45°C. Mix until dissolved. For PIF, test 10 × 25 g (total 250 g).

   ⏱ Time: 15 min • 🌡 Temperature: 45°C

2. Pre-enrichment in BPW

   Incubate suspension at 37 ± 1°C for 18 ± 2 h. Resuscitation phase — damaged Cronobacter cells regain growth capability.

   ⏱ Time: 18 h • 🌡 Temperature: 37°C

3. Selective enrichment in mLST/vancomycin

   Transfer 0.1 mL enriched culture to 10 mL mLST broth with vancomycin (10 mg/L). Incubate 44 ± 1°C for 24 ± 2 h.

   ⏱ Time: 24 h • 🌡 Temperature: 44°C

4. Isolation on CCI agar

   From yellowed mLST broth, streak onto CCI agar plate. Incubate 44 ± 1°C for 24 ± 2 h.

   ⏱ Time: 24 h • 🌡 Temperature: 44°C

5. Colony reading

   Look for typical Cronobacter colonies — blue-green (turquoise), diameter 1–2 mm. Other Enterobacteriaceae may grow as colorless or pink colonies.

   ⏱ Time: 15 min

6. Preliminary confirmation

   Select min. 5 typical colonies. Subculture on TSA and incubate 25°C for 48–72 h. Cronobacter forms yellow/yellow-orange colonies (pigment production).

   ⏱ Time: 48–72 h • 🌡 Temperature: 25°C

7. Biochemical identification

   Confirm isolates using API 20E, VITEK 2 GN or molecular methods (PCR). Cronobacter: oxidase (−), catalase (+), VP (+), arginine dihydrolase (−).

   ⏱ Time: 24–48 h

8. Reporting

   Result: "Cronobacter spp. detected/not detected in 25 g (or 10 × 25 g) sample". Detection in PIF = immediate notification of supervisory authorities and batch withdrawal.

   ⏱ Time: 10 min

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Cronobacter — BSL-2 pathogen, particularly dangerous for newborns and immunosuppressed persons
• Work exclusively in class II laminar flow cabinet — strict aseptic principles observance
• Vancomycin — potentially allergenic antibiotic, use gloves
• Autoclave used materials before disposal (121°C/20 min)
• Surface disinfection with 70% ethanol or 0.5% sodium hypochlorite

Frequently asked questions