In this section, you will explore the following questions:
- How do single-celled yeasts use cell signaling to communicate with each other?
- How does quorum sensing allow some bacteria to form biofilms?
Connection for AP® Courses
Cell signaling allows bacteria to respond to environmental cues, such as nutrient levels and quorum sensing (cell density). Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. For example, budding yeasts often release mating factors that enable them to participate in a process that is similar to sexual reproduction.
Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.
|Big Idea 3||Living systems store, retrieve, transmit and respond to information essential to life processes.|
|Enduring Understanding 3.D||Cells communicate by generating, transmitting and receiving chemical signals.|
|Essential Knowledge||3.D.1 Cell communication processes share common features that reflect a shared evolutionary history.|
|Science Practice||1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain.|
|Learning Objective||3.36 The student is able to describe a model that expresses the key elements of signal transduction pathways by which a signal is converted to a cellular response.|
|Essential Knowledge||3.D.1 Cell communication processes share common features that reflect a shared evolutionary history.|
|Science Practice||6.1 The student can justify claims with evidence.|
|Learning Objective||3.37 The student is able to justify claims based on scientific evidence that changes in signal transduction pathways can alter cellular response.|
Unicellular organisms were assumed to communicate at a very primitive level, but current research reveals the existence more complex signaling systems. Examples of these forms of communication are the formation of biofilms and quorum sensing. Biofilms have received the attention of researchers only recently for several historical and technical reasons. Since the germ theory of disease was established, the interest had been to isolate and characterize pathogens, not to study microorganisms as a community.
It is much easier to grow bacteria as pure cultures than replicate mixed populations biofilms, making the latter difficult to study in the laboratory setting. Such slime layers, previously considered haphazard assemblies of microorganisms, have been found to be highly organized ecosystems. The slime layer is made of extracellular polymers crisscrossed with channels for gases, nutrients, waste exchanges. Microbes attach to the solid substrate in a succession of populations.
Quorum sensing exists both within a same species and across species. It allows microbes to behave as multicellular populations and coordinate responses. One such example is the expression of genes encoding toxins in Staphylococcus aureus. Dr. Bonnie Bassler presents quorum sensing communication in Vibrio harveyi in this Ted Talk. Her enthusiasm and clear explanations make this video a thoroughly engaging experience. This is an opportunity to show a strong female role model in science.
Also available is this video clip: Quorum sensing molecules presented by Dr. Bonnie Bassler:
And an animation on quorum sensing in Vibrio harveyi can be found here.
Further reading: Painter, Kimberley L. et al. (2014). What role does the quorum-sensing accessory gene regulator system play during Staphylococcus aureus bacteremia? Trends in Microbiology 22:676–685
The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 3.31][APLO 3.37]
Within-cell signaling allows bacteria to respond to environmental cues, such as nutrient levels. Some single-celled organisms also release molecules to signal to each other.
Signaling in Yeast
Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. Budding yeasts (Figure 9.16) are able to participate in a process that is similar to sexual reproduction that entails two haploid cells (cells with one-half the normal number of chromosomes) combining to form a diploid cell (a cell with two sets of each chromosome, which is what normal body cells contain). In order to find another haploid yeast cell that is prepared to mate, budding yeasts secrete a signaling molecule called mating factor. When mating factor binds to cell-surface receptors in other yeast cells that are nearby, they stop their normal growth cycles and initiate a cell signaling cascade that includes protein kinases and GTP-binding proteins that are similar to G-proteins.
Signaling in Bacteria
Signaling in bacteria enables bacteria to monitor extracellular conditions, ensure that there are sufficient amounts of nutrients, and ensure that hazardous situations are avoided. There are circumstances, however, when bacteria communicate with each other.
The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reaches a certain level, specific gene expression is initiated, and the bacteria produce bioluminescent proteins that emit light. Because the number of cells present in the environment (cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing. In politics and business, a quorum is the minimum number of members required to be present to vote on an issue.
Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules such as acyl-homoserine lactone (AHL) or larger peptide-based molecules; each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off (Figure 9.17). The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers.
- Autoinducers must bind to receptors to turn on transcription of genes responsible for the production of more autoinducers.
- Autoinducers can only act on a different cell. It cannot act on the cell in which it is made.
- Autoinducers turn on genes that enable the bacteria to form a biofilm.
- The receptor stays in the bacterial cell, but the autoinducers diffuse out.
Some species of bacteria that use quorum sensing form biofilms, complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that will attack the host. Bacterial biofilms (Figure 9.18) can sometimes be found on medical equipment; when biofilms invade implants such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.
The ability of certain bacteria to form biofilms has evolved because of a selection of genes that enable cell-cell communication confers an evolutionary advantage. When bacterial colonies form biofilms, they create barriers that prevent toxins and antibacterial drugs from affecting the population living in the biofilm. As a result, these populations are more likely to survive, even in the presence of antibacterial agents. This often means that bacteria living in biofilms have higher fitness than bacteria living on their own.
Why is signaling in multicellular organisms more complicated than signaling in single-celled organisms such as microbes?
This question is an application of LO 3.36 and Science Practice 1.5 because students are describing and comparing models of signaling pathways in different types of organisms.
- The squid provides certain nutrients that allow the bacteria to luminesce.
- The squid produces the luminescent luciferase enzyme, so bacteria living outside the squid do not luminesce.
- The ability to luminesce does not benefit free-living bacteria, so free-living bacteria do not produce luciferase.
- Luciferase is toxic to free-living bacteria, so free-living bacteria do not produce this enzyme.
Research on the details of quorum sensing has led to advances in growing bacteria for industrial purposes. Recent discoveries suggest that it may be possible to exploit bacterial signaling pathways to control bacterial growth; this process could replace or supplement antibiotics that are no longer effective in certain situations.
Watch geneticist Bonnie Bassler discuss her discovery of quorum sensing in biofilm bacteria in squid.
- Bacteria interact by physical signals among a colony.
- Bacterium interact by chemical signals when it is alone.
- Bacterium interact by physical signals when it is alone.
- Bacteria interact by chemical signals among a colony.
The first life on our planet consisted of single-celled prokaryotic organisms that had limited interaction with each other. While some external signaling occurs between different species of single-celled organisms, the majority of signaling within bacteria and yeasts concerns only other members of the same species. The evolution of cellular communication is an absolute necessity for the development of multicellular organisms, and this innovation is thought to have required approximately 2.5 billion years to appear in early life forms.
Yeasts are single-celled eukaryotes, and therefore have a nucleus and organelles characteristic of more complex life forms. Comparisons of the genomes of yeasts, nematode worms, fruit flies, and humans illustrate the evolution of increasingly complex signaling systems that allow for the efficient inner workings that keep humans and other complex life forms functioning correctly.
Kinases are a major component of cellular communication, and studies of these enzymes illustrate the evolutionary connectivity of different species. Yeasts have 130 types of kinases. More complex organisms such as nematode worms and fruit flies have 454 and 239 kinases, respectively. Of the 130 kinase types in yeast, 97 belong to the 55 subfamilies of kinases that are found in other eukaryotic organisms. The only obvious deficiency seen in yeasts is the complete absence of tyrosine kinases. It is hypothesized that phosphorylation of tyrosine residues is needed to control the more sophisticated functions of development, differentiation, and cellular communication used in multicellular organisms.
Because yeasts contain many of the same classes of signaling proteins as humans, these organisms are ideal for studying signaling cascades. Yeasts multiply quickly and are much simpler organisms than humans or other multicellular animals. Therefore, the signaling cascades are also simpler and easier to study, although they contain similar counterparts to human signaling. 2
- The tyrosine kinases evolved before yeast diverged from other eukaryotes, but the other fifty-five subfamilies of kinases evolved after yeast diverged.
- Fifty-five subfamilies of kinases evolved before yeast diverged from other eukaryotes, but the tyrosine kinases evolved after yeast diverged.
- All kinases evolved in yeast, but yeast later lost the tyrosine kinases because they do not need them.
- The evolution of tyrosine kinases involved in cellular communication occurred about 2.5 billion years ago.
Watch this collection of interview clips with biofilm researchers in “What Are Bacterial Biofilms?”
- Bacteria often form biofilms in recurrent infections and these may be more antibiotic resistant.
- Bacteria rarely form biofilms in recurrent infections, making them more resistant to antibiotics than if they were not in a biofilm.
- Bacteria produce biofilms, which behave like a unicellular organism.
- Bacteria don't produce biofilms in recurrent infections but become resistant due to repeated exposure to antibiotics.
- 2G. Manning, G.D. Plowman, T. Hunter, S. Sudarsanam, “Evolution of Protein Kinase Signaling from Yeast to Man,” Trends in Biochemical Sciences 27, no. 10 (2002): 514–520.