Microbial Contamination in Water-Based Metalworking Fluids

Water-based metalworking fluids (MWFs) are highly susceptible to microbial contamination, primarily from bacteria and fungi, with over a hundred species identified. In deteriorated fluids, microbial concentrations often reach levels of 10⁷ to 10⁸ CFU/mL. Due to their strong environmental adaptability, most microbes can adjust their metabolic pathways based on nutrient availability. This flexibility enables certain dominant microbial populations to eventually prevail under specific environmental parameters.

|| Note: This article focuses on bacteria and fungi. Viruses and aquatic organisms are not covered.

Bacteria found in MWFs include species from the genera Pseudomonas, Aerococcus, Staphylococcus, Streptococcus, Alcaligenes, Corynebacterium, Proteus, Desulfovibrio, Salmonella, and Shigella. Among these, Pseudomonas is the most prevalent.

Some of the bacteria found in corrupted metalworking fluids

Common waterborne bacteria such as Pseudomonas aeruginosa, Enterobacter cloacae, Escherichia coli, and Klebsiella pneumoniae are frequently detected in MWFs. Under suitable conditions, bacterial populations grow exponentially—aerobic bacteria may double every 20–30 minutes, while anaerobes may double every hour.

Aerobic bacterial activity depletes oxygen in the fluid, paving the way for anaerobic bacteria. A putrid odor, often detected in deteriorated fluids, indicates microbial spoilage. One notorious contributor is the sulfate-reducing bacteria (SRB), which reduce sulfate and other sulfur compounds to hydrogen sulfide (H₂S)—responsible for the “rotten egg” smell.

SRBs include:

• Desulfovibrio

• Desulfotomaculum

• Desulfomonas

• Desulfococcus

• Desulfobacter

• Desulfobulbus

• Desulfosarcina

• Desulfonema, among others

These bacteria are strictly anaerobic, metabolically diverse, and thrive at 25–35°C, tolerating up to 55–70°C, with an optimal pH of 7.0–7.5.

Common forms of sulfate-reducing bacteria

Image source: http://hardsoftwater.com/sulfate-reducing-bacteria-test-kit/

In most cases, bacterial contamination is the primary cause of microbial growth and odor in MWFs, and bacterial counts are often used as an indicator of microbial contamination levels.

Fungi are classified into molds (filamentous fungi) and yeasts (unicellular fungi), each adopting distinct survival strategies within MWF systems:

• Molds form fuzzy or woolly colonies via hyphal growth. They attach to tank walls, form biofilms, and secrete acidic metabolites that corrode equipment. Mold hyphae also clog filters.

• Yeasts reproduce by budding and proliferate in the fluid phase, causing turbidity and altering surface tension. Certain yeast strains produce allergenic aerosols.

When both coexist, their competition creates a dual-phase damage model: molds colonize surfaces, while yeasts thrive in the liquid. Fungal biofilms and clusters are often found on:

• Tank walls

• Outlet valves

• Filter screens

• Pipelines

• Fluid surfaces

Although fungi are usually less concentrated than bacteria, they are harder to eliminate. Spores make them more resistant to biocides. Once mold overgrowth occurs, the entire system must be flushed with antifungal solutions and thoroughly cleaned to avoid recontamination.

Fungal aggregates – mycelium

Water-based MWFs form complex ecosystems with dynamic microbial communities. These microbes interact competitively and synergistically, influencing fluid performance and stability. Therefore, microbial control is a key part of MWF management.

Microbial contamination can:

• Alter fluid composition and properties

• Reduce tool and equipment life

• Pose health risks to workers

Monitoring microbial growth by measuring changes in population size or biomass over time is critical. This data helps evaluate:

• The effect of nutrients and environmental conditions

• The efficacy of antimicrobial agents

• Microbial growth behavior

Effective microbial control relies on understanding these dynamics and using this data to guide biocide use and system maintenance.

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