Microorganisms respond to a multitude of external stimuli in their habitat to select a suitable niche for survival and reproduction. Light and gravity are probably the most important cues for most motile microorganisms. Several types of light-induced behavior can be distinguished in microorganisms, including phototaxis, photophobic responses, and photokinesis. Chemical gradients, such as oxygen or carbon dioxide, are sensed by many organisms. Bacteria recognize and follow gradients of attractants (e.g., nutrients such as sugars) or avoid sources of toxins (e.g., phenol). Heterotrophic eukaryotic microorganisms also use chemical gradients to find their food. Pheromones are produced and emitted to attract gametes of the opposite sex. Some prokaryotic and eukaryotic organisms are capable of sensing extremely small thermal gradients very close to the physical limits. Almost all motile organisms, from bacteria to vertebrates, recognize and use the magnetic field of the Earth. Responses to electrical fields are not easy to explain, because these stimuli are not expected in nature. The responses to multiple stimuli may be additive or connected in a complex network of signal transduction chains. In other cases, responses to certain stimuli may override those to others.

Introduction

Microorganisms respond to a host of stimuli in their environment to search for and stay at favorable habitats optimal for their growth and survival, as well as reproduction. Their responses to these environmental clues may vary depending on their developmental stage.