Exploring the biological properties of bacterial monocultures and their co-cultured variations in Enterobacter, Citrobacter, Serratia, and Proteus species.
Beyond the world visible to the naked eye exists an invisible universe of microorganisms. Bacteria of the genera Enterobacter, Citrobacter, Serratia, and Proteus — typical representatives of the Enterobacteriaceae family — are most often mentioned in the context of infectious diseases. However, these microscopic organisms lead a much more complex and organized life than might appear at first glance.
When they exist alone (as monocultures), their properties are well studied. But the real mysteries begin when different bacterial species find themselves together in one environment — their coexistence radically changes the biological properties, behavior, and pathogenicity of each participant in this invisible alliance.
The Enterobacteriaceae family represents gram-negative rod-shaped bacteria that are widely distributed in the environment and are often part of the normal intestinal flora of animals and humans 1 . They belong to the class Gammaproteobacteria and include more than 68 genera with 355 species 1 .
These bacteria are facultative anaerobes (can live both in the presence and absence of oxygen), do not form spores, and usually have flagella for movement, with the exception of some species of Klebsiella and Shigella 1 .
Each genus of bacteria has unique characteristics that determine its behavior both in monoculture and in mixed communities.
| Bacterial Genus | Motility | Biofilm Formation | Key Pathogenic Properties |
|---|---|---|---|
| Proteus | Active "swarming" | Moderate ability | Urease activity, kidney stone formation |
| Serratia | Motile | Intensive formation of filamentous biofilms | Quorum sensing regulation, antibiotic resistance |
| Citrobacter | Motile | Capable of biofilm formation | Tropism for nervous system, intracellular survival |
| Enterobacter | Motile | Capable of biofilm formation | Nosocomial infections, antibiotic resistance |
When different bacterial species find themselves together in one environment, their properties can change radically. This occurs thanks to various mechanisms of intermicrobial interaction:
The ability of bacteria to coordinate behavior through chemical signals (acylated homoserine lactones), which plays a key role in the formation of mixed biofilms 3 .
When metabolic products of some bacteria serve as nutrients for others 2 .
For example, the ability of Proteus mirabilis to move non-motile bacteria such as Klebsiella pneumoniae along catheter surfaces 7 .
One illustrative example of the influence of co-culturing on the biological properties of bacteria is a study that examined the role of Citrobacter in increasing energy storage in fish on a high-fat diet 2 .
The study demonstrated several important effects of co-culturing:
The Citrobacter S1 strain successfully colonized the fish intestine, with its numbers significantly increasing in the high-fat diet group 2 .
Addition of S1 bacteria to the high-fat diet significantly increased body lipid content in fish compared to all other groups 2 . This was accompanied by increased triglyceride absorption efficiency and intestinal permeability.
This research demonstrates that even non-dominant bacteria in the intestine, such as Citrobacter, can significantly influence host physiology, especially in conditions of coexistence with other members of the microbiome 2 .
| Parameter | Control | Control + S1 | High-Fat Diet | High-Fat Diet + S1 |
|---|---|---|---|---|
| Body Lipid Content | Low | Low | Moderate | Significantly Increased |
| Triglyceride Absorption Efficiency | Basal level | Slight increase | Moderate increase | Significant increase |
| Citrobacter Count in Intestine | Low | Moderate | Moderate | High |
| Relative Firmicutes Content | 5.11% | 23.98% | 58.15% | 62.64% |
Biofilms are complex communities of microorganisms enclosed in a matrix of extracellular polymers synthesized by the microorganisms themselves 6 . This matrix provides protection from adverse environmental conditions, antibiotic actions, and the host's immune system .
A 2020 study analyzing Enterobacteriaceae strains from meat products showed that out of 200 strains, 46 (23.0%) were strong biofilm producers, 60 (30.0%) were moderate, and 79 (39.5%) were weak 6 .
The problem of antibiotic resistance among Enterobacteriaceae has reached alarming proportions. A 2020 study demonstrated that 63% of Enterobacteriaceae strains isolated from meat products were multidrug-resistant (resistant to two or more antibiotics) 6 . The greatest resistance was observed to ampicillin, cefotaxime, ceftazidime, and streptomycin 6 .
| Antibiotic | Percentage of Resistant Strains | Medical Significance |
|---|---|---|
| Ampicillin | 65.5% | Critically important antimicrobial |
| Cefotaxime | 54.5% | 3rd generation cephalosporin |
| Ceftazidime | 52.0% | 3rd generation cephalosporin |
| Streptomycin | 50.0% | Aminoglycoside |
| Tetracycline | 35.5% | Broad spectrum |
| Nalidixic Acid | 30.0% | Quinolone |
| Gentamicin | 18.5% | Aminoglycoside |
| Ciprofloxacin | 16.5% | Fluoroquinolone |
Modern research on bacterial co-culturing requires the use of various methods and reagents:
Allows visualization of biofilm structure and arrangement of different bacterial species within them 8 .
Computer modeling of interactions between potential antibacterial compounds and targets in bacterial cells .
Determination of the complete genomic composition of microbial communities 8 .
Assessment of bacterial viability and physiological state .
Instruments for measuring fluorescence, used to study molecular interactions 8 .
The study of biological properties of bacterial monocultures and their co-cultured variations opens new horizons in understanding the microbial world. It turns out that bacteria are not solitary players, but participants in complex communities where interaction between different species can radically change their properties, pathogenicity, and treatment resistance.
Understanding these mechanisms is necessary for developing new approaches in combating infectious diseases, especially in an era of growing antibiotic resistance. Perhaps the key to solving the problem is not destroying bacteria, but managing their interactions, disrupting communication between them, and targeted impact on mechanisms of collective behavior.
Invisible alliances of bacteria continue to be one of the most fascinating mysteries of microbiology, the unraveling of which could lead to revolutionary discoveries in medicine and biotechnology.